XHTML was developed by the W3C to help web developers make the transition from HTML to XML. So if you want to learn XML, XHTML is a good place to start. Once you start to learn XML, some of the concepts will already be familiar to you.

XHTML Tutorial
Learn about the differences between HTML and XHTML. It's not that hard really!

W3C Recommendation (W3C)

Introduction to XHTML, with eXamples (WDVL)

Why do we need XHTML? What's wrong with HTML? (WDVL)

XHTML Validation Service (W3C)