XML

Extensible Markup Language is a recommendation from the World Wide Web Consortium for a more powerful language for the description of shared data. Whereas HTML is restricted to describing the format of transmitted data, XML allows transmitting the data in a format-independent manner so that the receiving computer can exert more control over the way in which the data is processed and presented. XML can be used by any individual or group of individuals or companies that wants to share information in a consistent way.

W3C XML
Cover Pages reports on SGML/XML
IBM XML Developers
IBM XML tutorial