XML is not much - this is its strength.
All XML does is provide a nice flexible internationalized way to label the elements of a data structure and ship them around with the labels attached.
(Tim Bray)
XML is a metalanguage, from which new languages are created.
XML can be seen as a set of basic rules that describe requirements for the syntax of the new languages; often called "XML Grammars".
Listing of available XML grammars as of April 19, 2003 (www.kensall.com)