XHTML is basically HTML written they way it should be written, with a strict format.  There are a few little things about XHTML that you need to know about to write XHTML compliant documents, but for the most part it's just the matter of having well formed HTML.

What does well formed mean.  We'll it means that all your markup tags are closed, there are no overlapping markup tags, all attributes names are lower case, and their names are inclosed in quotes.  You also have to make sure that HTML entities such as < > & are HTML escaped using their HTML codes like &lt; &gt; &amp;

For the most part, that's about it.  What is the advantage of having a website that is XHTML compliant?  It means that your site will display properly in different browsers including alternative browsers such as speech readers.
Valid XHTML 1.0 Transitional