What is XHTML?
XHTML is one of the current W3 recommendations for Web page authoring. It is basically a reformation of HTML as an XML application... in layman's terms it is a stricter version of HTML.
The term XHTML actually stands for eXtensible HyperText Markup Language and can be considered as HTML 5.0. XHTML is as easy to understand and learn as HTML 4, so if you have already got some knowledge of how to use HTML you should have no problem learning XHTML. The main difference between the two languages is that in XHTML all tags must be closed (so you can forget about leaving those paragraphs open!), but the fact is that XHTML is a much stricter language and so is more likely to produce the same results on different platforms! This whole site is written in XHTML!
There are a few minor differences between XHTML and HTML 4.01, and a more detailed explanation of these differences can be found on the XHTML differences page.