Because HTML is an evolving language, there are many versions of it. Visit the XHTML entry on Wikipedia for the latest information as it is constantly being updated.
HTML 4.0 is deprecated which means that it should not be used.
The version after HTML4.0 is XHTML1.0. The reason for the new name is that it is a reformulation of HTML4.0 in XML format. Hence the "X" in "XHTML". XHTML is what is being in use currently by the majority of web community as of 2008. However, people have been calling "HTML" for so long that people still are loosely using the term "HTML" even when they are really referring to "XHTML". Learn more about the difference between HTML and XHTML read the Sitepoint article here or watch the lynda.com movie here.
The current version of XHTML as of June 2008 is XHTML 1.1.
There are other versions in the works such as HTML5 and XHTML2.0
