This is a super-simple, but great overview of XML Namespaces
http://www.w3schools.com/xml/xml_namespaces.asp
This article is great for understanding why they exist.
http://msdn.microsoft.com/en-us/magazine/cc302166.aspx
The W3C formal definition (if you ever have problems falling asleep)
http://www.w3.org/TR/REC-xml-names/[/url]
Edit: Make my links clickable :w00t:
-- Itzik Ben-Gan 2001