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Some General Issues

There are many formats for exchanging and storing information. Some of these cover information which can be viewed but not edited. Other formats are concerned with the information (numbers, text, images) but make no reference to how the information is to be viewed or laid out. In the latter case the way the information appears to the user is a function of how it is viewed, for example on paper or on the screen. Some formats are appropriate for text and others for pictures. Some offer compact methods of storage and others less so. It is important to appreciate these differences when choosing a format for storage, particularly for archiving purposes where the eventual use to which the information is to be put may not be known.

One could group format types in various ways. It is useful in the context of the eLib programme to identify the following types:

• Simple ASCII text with printing characters and no formatting other than spaces, tabs and line structure. This is unlikely to be of major importance within the eLib programme and is not discussed further.
• Proprietary 'closed' word processing formats. These are not encouraged within the context of the eLib programme and are not discussed further.
• Graphics formats, for example TIFF, GIF and PNG.
• Page description formats, for example PostScript and PDF.
• Files containing information about layout and content, for example RTF and TeX.
• Structured information where layout is a separate exercise, for example SGML.
• Formats for document delivery
• Moving images, 3D information and audio.

In addition this section includes some treatment of:

• Character sets
• Compressed data.

Graphics Formats

Images are stored generally in one of two fundamental formats, a raster format or a vector format. Raster formats essentially store the pixels in the raster lines of the image and may contain any number of colours (fixed by the individual format). The colour space of a format is the model that internally represents the colours within the image (up to a set maximum) based on combinations of some primitives, such as triples of red, green and blue (the RGB colour space) or triples of hue, saturation and value (the HSV colour space). TASI (Technical Advisory Service on Images)recommends that the images remain in the capture colour space and are only converted as needed. Raster formats are the most popular to be found on the Internet, but can suffer from a number of disadvantages: 'jagged' edges, poor resolution, the treatment of text information as image data and the inability to scale without distortion. A number of common raster formats are described below.

GIF

The Graphics Interchange Format (GIF) format defines a protocol which supports the hardware independent, online transmission of Raster graphics data (i.e. images). It uses a version of the LZW compression algorithm for its compression.

GIF is defined in terms of data streams which in turn are composed of blocks and sub-blocks representing images and graphics, together with the essential control information required in order to render the resultant image on the target output device. The format is defined on the assumption that an error-free transport level protocol is used for communication i.e. no error detection facilities are provided.

GIF has recently gained in popularity owing to its extensive use in the Web.

Although GIF is subject to a patent claim by Unisys, they have granted a limited, non-exclusive, royalty-free license for its use in certain computer software viewers.

JPEG

The need for standards which can represent grey scale and colour images in an economical and efficient way was acknowledged by the setting up of the Joint Photographic Expert Group (JPEG). This is a joint committee of ISO and CCITT. The JPEG standard is an ongoing project within ISO and CCITT as more efficient algorithms are sought for future versions of the standard.

The ISO standards covering the JPEG format define a number of different types of JPEG: "baseline JPEG", "lossless JPEG" and the recently introduced "JPEG-LS". "Baseline JPEG" is by far the most popular format in use at present. "Lossless JPEG" has become virtually obsolete and it remains to be seen how "JPEG-LS" (a "lossless" format) will be adopted for use on the Internet.

"Baseline JPEG", although able to achive visual indistinguishability from its original photo, is mathematically "lossy". The lossy compression is designed to take advantage of weaknesses in the human visual system to store detailed pictures such as "real-life" scenes (rather than, say, line-art). The degree of loss of data can be controlled by choice of parameters which are defined in the JPEG specification.

PNG

The Portable Network Graphics (PNG - 'ping') format is designed to be a successor to GIF. It was developed during 1995 and has been adopted by CompuServe to replace GIF. From the web perspective, PNG has a number of advantages over GIF including: alpha channels (variable transparency), gamma correction (cross-platform control of image brightness), two-dimensional interlacing and slightly better compression. PNG supports pallet-based images (8-bit), up to 48-bit ('truecolor') RGB or 16-bit greyscale. Unlike baseline JPEG, saving and re-saving a PNG image will not degrade its quality, making it a useful format for storage at intermediate stages of editing. Final edits of photographic images are probably better converted to JPEG. In constrast to GIF, multiple image support is not a feature of PNG. However. a PNG-like extension format called MNG is currently in development.

TIFF

Aldus Corporation designed and made public the Tagged Image File Format (TIFF) in 1986. Although initially targeted at desktop publishing applications, it has been widely implemented on many computing platforms and has become a de-facto industry standard format.

TIFF is a a tag-based format for storing and interchanging raster images. Typically TIFF describes data that comes from scanners, frame grabbers and paint programs. It is capable of describing bilevel, grayscale, pallet-color and full-color image data in several color spaces. There are four basic TIFF internal formats, bitmap, grey-scale, pallete-color and RGB-color. Each of these internal formats supports its own options for compression.

TIFF is widely used but it should be noted that there are problems with different versions and different flavours with people adding their own tags which interpreters do not necessarily understand.

PhotoCD

PhotoCD is a proprietary format devised by Kodak for the digital storage of constrained resolution images on CD. These images can be viewed at a number of resolutions and can be manipulated using image processing software. This format tends to use the PhotoYCC colour space.

Group 4 fax

CCITT have developed a series of compression mechanisms for transmitting black and white images across telephone lines which are used by fax machines. The standards are officially known as CCITT Recommendations T.4 and T.6 but are more commonly known as Group 3 and Group 4 compression respectively. Group 4 fax is likely to be used in some eLib document delivery projects.

Vector formats

The alternative to raster formats, vector formats, can result in smaller files and better representation. CGM is the International Standard for storage and exchange of 2D graphical data. Although initially a vector format, it has been extended to include raster capabilities. Formats likely to be heard of include the WMF (Windows Meta File) format found on PCs that supports Version 1 of CGM:1992 and the PGML vector format that has recently been submitted to the W3C. Common web browsers do not currently contain native support for vector formats.

Page Description Formats

Introduction

There are document formats which seek to preserve the image of an entire printed page. The page can be stored using a page description language (PDL) where the text and graphics are stored together with output information such as layout, font, linestyle etc. This means that more information is stored than within a Raster format. PDLs are designed for presentation of complete, formatted, final-form (non-revisable) page images on output printing devices. 'Virtual paper' is a good metaphor for PDLs. Most PDLs are oriented toward presentation of pages on laser printers. PostScript and the Portable Document Format (PDF) are described below as these are likely to be important within the eLib programme.

PostScript

PostScript is a Page Description Language (PDL) designed by Adobe Systems Inc. PostScript is the most successful of the commercial PDLs and has had a heavy influence on the final appearance of the Standardised Page Description Language (SPDL, an ISO standard). It offers the advantages of potentially high resolution colour output - that is, it is as good as a printed paper copy - and this means more flexibility in that the scale can be changed without loss of information.

As the 'language' part of PDL suggests, PostScript is a true interpretive programming language. Its Level-1 definition includes over 270 operators, which go far beyond basic graphics presentation (definition and maintenance of 'dictionaries', boolean and arithmetic operators, etc.). The more recently released Level-2 definition contains over 420 operators.

PostScript uses programming language constructs and paradigms: procedures, variables, conditional logic, etc. This creates powerful expressive capabilities. The trade off is that, compared to more object-oriented graphics formats, a PostScript graphics file is very difficult and impractical to edit or modify. Although device-independent, the PostScript imaging model demands Raster devices for presentation. The language is implemented on powerful onboard micro processors on many Raster devices (PostScript demands a lot of memory and computational power to interpret).

Encapsulated PostScript (EPS) is a (large) subset of PostScript which allows storage of information in the PostScript language but excludes any size or positioning information. This means that a part of a page can be brought in to another document. This is most frequently used for the inclusion of graphics within documents where these have been produced a different package than the one used for producing the text.

PostScript Level 2 is likely to be useful within the eLib programme for delivery of high quality page descriptions which are for printing only (although equivalence cannot be guaranteed across printers). Where page fidelity is needed together with the ability to annotate and interact with a document then PDF, discussed below, is likely to be the format needed.

Portable document format and Acrobat

Adobe have further developed the PostScript concept to define their Portable Document Format (PDF) which links with a suite of software called Acrobat. PDF and Acrobat are likely to be important within eLib. This position has been strengthened by the fact that there is a freely available, reader, the latter version of which have support for motion video.

PDF extends PostScript Level 2 to allow the addition of links within and between documents, annotations, thumbnails of pages, and chapter outlines which link to specific pages for access. The basic metaphor again is the page. This can be very attractive to publishers who wish to define a house style or who wish to have an online version of a paper journal. One such example is the Electronic Publishing Journal from Wiley which is described in Smith et al (1994).

The Acrobat software has a number of modules. These are:

• Acrobat Reader
• Acrobat PDF Writer to interface to other software
• Acrobat Exchange which allows some editing and permits annotations, links and security controls to documents
• Acrobat Search for searching documents
• Acrobat Type Manager which ensures suitable use of fonts and resolution for devices
• Acrobat Distiller for conversion from PostScript or EPS
• Adobe Acrobat for Workgroups to enable co-operative working on documents
• Acrobat Catalog for creating indexes for subsequent use in the Search package
• Acrobat Capture for interfacing to scanners and enabling the scanned page to be converted into PDF. This does character recognition and also maps the printed font into an Adobe font

This software is available for the PC, Macintosh and Unix platforms. Adobe have indicated that they intend to extend PDF to encompass SGML files, in order to be able to preserve structure within the file.

Further reading

• Smith, P.N. et al. Journal publishing with Acrobat - the CAJUN project. Electronic Publishing. Vol 6(4), 1-13, Dec 1993.
• Adobe Acrobat reader/plugin <URL:http://www.adobe.com/supportservice/custsupport/download.html>.

Formats containing layout and content

Another storage possibility is storing the layout of a document and the contents of that document separately. This is the approach taken by the Open Document Architecture (ODA) standard. In ODA the layout of the document is stored. This might include pages, title areas, places for pictures etc. The standard also allows the logical structure of the document to be defined. This may be chapters, paragraphs etc. which are then linked to the layout of the document. Alongside this the standard allows various content architectures to be positioned into places on the 'page' (this page could be a piece of paper or a screen and the layout may vary depending on the output medium). ODA standardises a number of content architectures. One of these is the Computer Graphics Metafile standard. Another is a Raster format based on the CCITT fax standard (see Annex A).

It is unclear what future there is for the ODA standard. It is not recommended for use in the eLib programme.

Rich Text Format (RTF), which can be output from and input to many word processing packages, is another example of this form. RTF is not a portable as many believe with problems even between different versions of the same software. It is possibly useful in private correspondence but not for general use in eLib projects.

LaTeX/TeX, used by many mathematicians and computer scientists, is another example of marked up text which contains information about both structure (paragraphs, mathematical symbols) and layout. As mentioned above, TeX is often used within other formats to enable the inclusion of mathematical symbols.

Structured information

Structured information contains embedded mark-up (often known as tags) which describes the information but which does not include any layout information.

SGML

The Standard Generalised Mark-up Language (SGML) provides a meta-language, or syntax, for writing rigorous, descriptive definitions of documents. It is independent of any system, device, language or application and allows individuals or groups of people in user communities to write their own types of documents within a standard framework. The information may include information beyond text and this may be image data stored in fax format or may be a CGM file. This standard separates document definition from subsequent access and viewing and allows information be accessed in ways not predicted at the time of mark-up.

Each SGML document contains three parts. The first is an SGML declaration which describes the environment in which the document needs to be processed and may include information about which character sets are to be used. The second part is the Document Type Definition (DTD) which describes the logical model for the document and defines references to entities which may be referenced, such as a fax image or CGM file. The third part is the document stream itself.

The SGML standard has an associated standard called the Document Style Semantics and Specification Language (DSSSL) which gives rules of presentation and style for the logical document components, for example headers, footers, chapter headings etc., which are defined in the document. The document may then be output via a page description language such as PostScript.

SGML is flexible. There is little restriction - too little many would argue - on the mark-up used. The standard is beginning to be taken on and used in publishing applications, by the US Department of Defence and by providers of some experimental online information services. It is gaining momentum in the marketplace especially as the multimedia standard called HyTime, which is an SGML application, seems to be being taken up by a number of key players in the market. The Text Encoding Initiative, a set of document type definitions produced within the academic community for the encoding of humanities research datasets, is also providing some impetus.

It is anticipated that SGML will be of use to several projects, even where materials may output to XML for delivery. Projects are encouraged to work together to agree or, where necessary, develop document type definitions. Several existing DTDs are likely to be relevant.

Text Encoding Initiative

The Text Encoding Initiative (TEI) is an international research project which aims to develop and disseminate guidelines for the encoding and interchange of machine readable texts within the humanities. These guidelines are available and offer a comprehensive mark-up capability using SGML which has application beyond the humanities.

HTML

Hypertext Markup Language (HTML) is simply a DTD which prescribes formats for presentation and display. Hypertext documents in the World Wide Web are written in HTML. eLib projects will make heavy use of HTML and should use HTML 3.2 and HTML 4.0. Microsoft, Netscape and other vendor-specific extensions are deprecated.

XML

The Extensible Markup Language (XML) is an application profile of SGML and was developed to address deficiencies in HTML. The extensible in XML obviates the need to propose additions to HTML or to use existing, but non-ideal, methods of markup. XML allows a markup structure to be tailored around the nature of the data to be represented. Previously SGML was the primary method of defining new document types, but although powerful, the syntax of SGML is complex. XML therefore introduces two new degrees of freedom to document creation on the web: freedom from an inflexible document type (HTML) and freedom from the complexity associated with defining new document types. Designed for ease of implementation and interoperability with HTML and SGML, the goal is that XML will enable its documents to be served, requested and processed in the way that HTML documents currently are.

An XML document essentially defines a data object that is composed of XML entities. An entity, like a variable, may store either text or binary data. Text within a document will either be a part of the data described or part of the markup. The markup encodes a definition of the document structure so that an XML processor will have access to the appropriate content and structure.

XML is a data object and has no concept of layout; in fact XML documents may not be designed for graphical rendering, being used primarily as a 'data-container'. On the Web however, it is likely that most XML documents will require a graphical representation and XSL was developed for this purpose. XSL, the XML Style Sheet Language, provides a method of defining layout, i.e. how a rendered XML document should appear in an XML browser.

Currently, browsers have only limited native support for XML (e.g. Internet Explorer 4.0 supports CDF, an XML application). Further support should be seen in the releases of Internet Explorer version 5 and Netscape version 5. To view XML documents with current browsers, an intermediate client plug-in program may be used, using Javascript, Java, ActiveX or similar. Another approach is server-side conversion of XML to HTML.

A number of XML applications (formalised XML document structures) exist. Popular applications include CDF (Channel Definition Format) developed by Microsoft for use with push technologies, CML (Chemical Markup Language) developed at Nottingham University, MathML (Maths Markup Language) a recent W3C recommendation and RDF (Resource Description Framework) a framework for describing metadata applications.

XML-related development is currently very active. This includes support technologies, from which XLink (providing a rich hyperlinking mechanism) and XPointer (providing access to components of XML resources) have recently emerged as well as application level technologies.

Further reading

The World Wide Web Consortium (W3C) is a useful place to start for information on HTML and XML.

• For information on HTML see <URL:http://www.w3.org/MarkUp/>.
• For information on XML see <URL:http://www.w3.org/XML/>.
• For information on SGML see <URL:http://www.w3.org/MarkUp/SGML/>.

Moving images

MPEG

The Moving Picture Expert Group is another joint ISO and CCITT group who aim to develop international standards for compression, decompression, processing and coded representation of video and associated audio. "MPEG video" currently consists of two standards, MPEG-1 and MPEG-2. Another, MPEG-4, is in the process of development. The one-time standard for HDTV, MPEG-3, was dropped since it was realised that MPEG-2 could cover the requirements.

The algorithms used for encoding and decoding may be implemented in software or in hardware (e.g. for decoding there are MPEG-decoder cards for PC's and dedicated hardware such as CD-i).

The MPEG-1 standard was released in 1992 optimised for the delivery of audio and video over low bit-rate channels (around 1.5Mbits/s) such as CD-ROMs (including Video-CD and CD-i) and digital telephone networks (T1's, Video on Demand and so on). The standard defines a system of dealing with video (based on H.261 [CCITT 1998-1990] and JPEG) and audio (based on MUSICAM technology). MPEG-1 was optimised to work at resolutions of 352x240 pixels at 30 frames/sec (NTSC) or 352x288 pixels at 25 frames/sec (PAL). MPEG-1 resolution is not however limited to these sizes and may go as high as 4095x4095 pixels at 60 frames/sec. Interlaced video applications (for example broadcast television) are not directly supported by MPEG-1. MPEG-1 is the standard used by a number of common (software-based) MPEG utilities. The strength of MPEG-1 is its high compression ratio with relatively high quality.

MPEG-2 supports a wider range of resolutions and bit-rates than MPEG-1 and was designed with a different purpose in mind, though is 'back-compatible' in that MPEG-2 decoders should play-back MPEG-1 streams. The MPEG-2 standard was finalised in 1994 and addressed the issue of interlaced-video and scalability. This creates the potential to represent high quality digital video - higher quality images at higher bit rates. MPEG-2 streams at lower MPEG-1 bit rates will have lower quality than an MPEG-1 stream, but at its specified bit-rates (between 3-10Mbits/sec), MPEG-2 at 720x486 pixels NTSC delivers true broadcast quality video. MPEG-2 is used chiefly by broadcast and cable companies who require broadcast quality digital video.

GIF89a

The GIF89A format allows multiple images to be encoded within a single file with a specified frame-flick rate. GIF89A's are supported without plug-ins by many web browsers and only loads once. The limitations include a maximum of 256 colours; audio is not supported. Photographs are better compressed by the JPEG method used in MPEGs.

AVI and QuickTime

AVI (Audio Video Interleaved), a.k.a VFW (Video for Windows) is a Microsoft proprietary container format. Typically, the quality is moderate, low VHS standard at 15fps with mono sound in a quarter screen window. However, different codecs can obtain different results, for example, Intel's Indeo4 can produce MPEG1 quality.

QuickTime was developed by Apple. Players or plugins are widely available for various platforms. QuickTime 3 opens MPEG. DV and AVI. The more fully-featured QuickTime tools are still limited to the Mac platform such as vector graphics, animation and 3D sprites.

Streaming Video

RealVideo ia a proprietary format optimised for streaming(i.e. played while the file is still downloading) over the internet. Real Networks claim near-CD quality audio. The quality is downgraded from a QuickTime movie. Performance is better with fast connections (higher than 56k) but it will work at 28.8.

VivoActive is another proprietary streaming format. Vivo claims FM-quality audio. Independent reviews claim good video quality for a streaming format.

MBONE

Mention should also be made of the MBONE, although it is a medium not a format. The MBONE stands for IP Multicast Backbone on the internet. IP-Multicast is the class-D (224.0.0.0 - 239.255.255.255) addressing scheme in IP. IP Multicast-based routing allows distributed applications to achieve time-critical "real-time" communications over wide area IP networks, such as audio/visual teleconferencing. Special routers ("mrouters") distribute multicast data-streams efficiently to subscribed hosts. Further information on the JANET MBONE service can be found at <URL:http://www.ja.net/documents/mbone.html>.

Three dimensional information

Many eLib projects will only be concerned with 2 dimensional pictures (even if they represent a 3 dimensional scene).

Various packages produce 3 dimensional files as output. Some of these relate to the CAD area though are more generally used for file exchange. IGES, AutoCAD DXF and PHIGS Archive files are examples of formats which exist but are probably of little relevance to the eLib programme.

Perhaps of more interest is VRML (Virtual Reality Modelling Language). This seems to be emerging as the standard for 3D objects within the Web. VRML is built on the Open Inventor technology from Silicon Graphics. VRML provides a graphical mark-up allowing links to other VRML and HTML resources. There are some public domain viewers.

Audio

A selection of sound format files popular on the internet are described below.

AU

A sound format developed by Sun/NeXT and possibly the most popular format for samples. Many utilities on a number of platforms support playing AU files. It is not highly compressed.

MP3

MP3 stands for MPEG audio layer 3. It is a highly compressed format (typically 1:12 for stereo files) which works by utilising weaknesses of human audio perception. Defined within the MPEG-1 and MPEG-2 standards, this format is popular where high quality (CD audio quality) reproduction of long samples is required. MP3s are also amenable to streaming and are supported by software on a number of platforms.

WAV

The full name for this format is "RIFF WAVE" and is a format developed by Microsoft and IBM. Its inclusion on Microsoft Windows machines has made it popular. Like AU files, WAVs are not highly compressed.

RAM/RM

The RealAudio streaming audio format. This provides (depending on the version) from AM-quality over 14.4 kbps upto near-CD quality over ISDN and LAN connections.

MIDI/MODs

Two types of format are popular for playing audio sequences, digital modules and MIDI. A digital module is a music file typically with the extension MOD, S3M, FAR or MTM. These formats contain musical patterns and a selection of sound samples, so that music can be reproduced from the patterns with the appropriate sample (instrument). MIDI files are essentially patterns and sequence information to drive a MIDI device. MIDI files may be played on a computer via a program or plugin which will emulate a number of the instruments defined in the MIDI standard.

Document delivery formats

The GEDI agreements are recommended for use in electronic document delivery projects.

This is the format for Generic Electronic Document Interchange (GEDI), overseen by ISO TC46/SC4 (the committee responsible for the standardization of computer applications in information and documentation). The working draft (Jan 1998) of this proposal suggests that files should be interchanged as images, using the TIFF, JPEG and PDF data interchange formats, and describes associated metadata. Information about GEDI can be tracked on the OII Standards Pages <URL:http://www2.echo.lu/oii/en/oiistand.html>

Character sets

If data is stored as a page image these characters can be stored in Raster format along with the text. This may be appropriate where there is no requirement (ever) to discover the meaning of the symbols (for example chemical or mathematical symbols) or text (which may include diacritics or a range of character sets, for example Kanji).

There are some other solutions. One is to use an extended character set which moves beyond the standard printing characters. ISO have been working in this area for many years as a number of projects require standards for character sets to underpin their work. ISO 646 was the first of these standards and defines 128 representations of characters for digital storage (using 7 bits to define each character). ASCII is the American definition of ISO 646 (that is it has the dollar as its currency symbol). ISO 2022 moves on from that in allowing the same number of characters to be defined at any one time but permits character set switching giving sufficient access to Latin alphabets but does not address non Latin alphabets at all. In recognising this need, ISO developed first of all a series of 8 bit character sets in ISO 8859 and more recently a 32 bit character set in ISO 10646. ISO 8859 defines a series of nine 256-character alphabets, each of which consists of the US ASCII characters in the first 128 positions and a series of accented characters or non-Roman characters in the second 128 positions.

Between them the ISO 8859 character sets cover the great majority of European languages, including both Roman and Cyrillic alphabets for eastern European languages, together with Greek, Hebrew and Arabic. The far greater number of characters in the ISO 10646 standard is intended to address all of the world's written languages, including the wide range of characters and ideograms used in Asian languages, but its use in practical applications is at present negligible and it should be regarded as a standard for the future.

It is also possible to use some form of 'markup' which uses printing characters in a pre-defined way which enables the interpreter to recognise that this is a special symbol. An example of such markup is used in the text processing language called TeX. This language is used by many mathematicians and computer scientists when writing papers as it allows the use of symbols such as integral sign, summation symbols and so on. XML will be very important in this area.

Work is currently taking place in the Internet standardisation process to incorporate chemical symbols within MIME and Web standards. HTML 4.0 also addresses the need for extended character sets.

Character sets for textual files transferred via FTP or mail should normally be restricted to those specified in the MIME standard, i.e. US-ASCII for 7-bit text and ISO 8859-n (n = 1-9) for 8-bit text. The use of ISO 8859-1, also commonly known as ISO Latin-1, is particularly commended wherever applicable in view of its widespread availability on end-user devices such as terminals, personal computer systems and printers. Use of other ISO 8859-n variants may well require provision of additional fonts and/or special software (e.g. for right-to-left languages) on many such devices.

Compression

Compression should be mentioned in this section due to the space (and bandwidth & time) advantages of storing (and transferring) compressed formats. An archiving utility will represent a directory hierarchy and its contents in a single file from which the original hierarchy and contents may be cloned. A compression utility utilises an algorithm to encode a file into a smaller one (typically 30-60% smaller) whilst retaining enough information to re-build the original file.

Different tools may use different algorithms and encodings, which means that the compressed data must be un-compressed with the complement of the compression tool. A number of common utilities exist to allow archiving and/or compression. Traditionally many were born as platform-specific and have tended to remain popular on that platform. A number of popular, cross-platform tools are described below with a bias towards those that originated on Unix servers.

compress

A widely used and available Unix compression utility. Compressed files are recognised by their .Z extension.

gzip

Initially a Unix compression utility but now available for other platforms. Compressed files are recognised by their .gz extension.

jar

A Java archive, a feature of JDK1.1 designed to be a platform independent format to transfer collections of Java files such as class files. Jar is very similar to tar (below) though is based around zip. Jar files have the extension .jar.

tar

An archiving utility commonly found on Unix systems. Archived files are recognised by their .tar extension. Often tar files are compressed and end up having a extensions such as tar.gz, .tgz or .tar.Z.

zip

An archiving and compression utility. Very common on PCs (pkzip, WinZip) but with counterparts on many platforms (including Unix zip). Zip files have the extension .zip. Note that zip and gzip are not synonymous.