The eXtensible Markup Language
Enables structured documents to be served, received
and processed on the Web in the way that is now possible with HTML
A markup language is a language for structuring a
textual document through the addition of tags. Tags behave
as labelled brackets, surronding text and other tags. The language
is completely defined by its tags and the rules for nesting them.
This is a trivial example.
XML is NOT a markup language, but a way of
describing new markup languages and documents in these languages
===> markup languages are instances of XML.
This is an XML Document Type Definition
(DTD) describing the XML language of the previous example.