XML

Acronym. eXtensible Markup Language. XML is a file format which is commonly used for:

Used in AJAX applications to communicate with a Web Server
Configuration files in Java applications
XHTML web documents (a standard of the W3C, related to HTML)
RSS feeds
XMLRPC Servers

XML and HTML are related, as they use a similar syntax, however, XML uses a much stricter syntax.

XML looks like HTML:

<products>
 <product name="dog food" upc="541231212567">
  <prices>
   <price currency="USD">53.33</price>
   <price currency="EUR">21.99</price>
  </prices>
  <name>Zug Zug Dog Food</name>
  <description>
   <![CDATA[
    This is the only kind of food that won't eat.
   ]]>
  </description>
 </product>
</products>

But it can be associated with a DTD (Data Type Definition) which then defines what tags and attributes are permitted.

For more information check out the XML specification on the W3C web site.