The WWW Consortium defines XML
" XML is primarily intended
to meet the requirements of large-scale Web content providers
for industry-specific markup, vendor-neutral data exchange,
media-independent publishing, one-on-one marketing, workflow
management in collaborative authoring environments, and the
processing of Web documents by intelligent clients."
What this really means is this
is a language that can be included on web pages that allow you
to create intelligent classifications for your web searches.
Thus a search for frogs won't bring up a page that says 'a frog
in your throat'.