Notes:
XML stands for extensible markup language
Based on SGML, so same “parent” as HTML
Unlike HTML, not fixed format with a predefined set of tags
You make the tags up as you go along
(Can use someone else's set of tags if you wish)
(Standard tags do exist in formats derived from XML, eg SOAP, WSDL)
XML is a metalanguage, ie a high-level concept, up to implementation to define the fine details.
XML is primarily for describing content or information. For example, you might have a headline:
England beat Denmark 3-0
In HTML, you'd mark it up as:
<h1>England beat Denmark 3-0</h1>
which means nothing other than “make it big”
In XML, you describe your content, so:
<headline>England beat Denmark 3-0</headline>