Screaming Penguin - Source Does Matter

developerWorks has a nice tutorial on XSL and XSLT. This tutorial is also has a good XML description and basis. Definitely worth a read, checkit via the links.

UPDATE: Webmonkey has an XSLT article too! Check it here.
Excerpt:

This article presents a simple, hands-on exercise that demonstrates the principles of the Extensible Stylesheet Language Transformations (XSLT). It takes about an hour to complete the concept exercises and about 15 minutes at a computer to try out the results with a real XSLT
processor.
  Hands on XSL: developerWorks