<p>What are the main differences between XHTML and HTML? Which one is better in your opinion, and why? Do most browsers support both?</p>

The Extensible Hypertext Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML, but also conforms to XML syntax.
XHTML is "the modern version of HTML 4".
More info :
wikipedia
and
W3C