In basic terms, Z39.50 is a network protocol* which allows searching of (usually remote) heterogeneous databases and retrieval of data, via one user interface. It is most often used for retrieving bibliographic records, although there are also some non-bibliographic implementations.
[*Protocol: a strict set of rules that govern the exchange of information between computer devices.]
Presentation slides from an introductory talk at a meeting organised by ASLIB in October 1997.
Z39.50-1995, or Version 3, is the latest version, approved by ANSI at the end of 1995. Technical development of the standard is taken forward by the Z39.50 Implementors' Group (ZIG), consisting of interested individuals from institutions and companies. The majority of these are North American, but there is an increasing number of European activists. (However, more are needed.)
Z39.50 version 3 was accepted as an ISO standard in March 1997. The new protocol will be known as ISO 23950.
To be continued :-)