XHTML (Extensible HyperText Markup Language)
XHTML is the new, cleaner and more precise version of HTML, made by combining HTML 4.01 and XML, taking the elements from HTML and the syntax from XML. It is almost identical to HTML 4.01, so there is not a lot of changes. XHTML was created to replace HTML completely.
W3C recommends using XHTML since January 2000. That means it is stable and checked by people from W3C organisation.
XML + HTML = XHTML
Generally, the most of today web pages on World Wide Web was written in "bad" HTML code, and XHTML is the one that prohibits "bad" coding. The code must be 100% correct and has to respect certain rules.
On today's market you have a lot of browsers, some of them are made for computers, some of them for mobile phones, and some of them for pocket computers. These last two have no resources nor power to interpret a "bad" markup.
That is why was created a markup that can be used today (PCs) and also that can be used in the future (mobile phones and pocket computers). That is one more reason why the XHTML was created, and that is the reason to start using it right away.
More about XHTML you can read at good old W3C.org.