a very fast introduction
- http://en.wikipedia.org/wiki/XML

  - http://www.w3schools.com/xml/xml_whatis.asp

 
Official specifications:
http://www.w3.org/standards/xml/

 and related arguments:
- http://www.w3.org/standards/xml/core
- http://www.w3.org/TR/2008/REC-xml-20081126/