XHTML is a different thing than HTML. XHTML is XML-based. HTML is SGML-based. It is similar, but since XHTML is based in XML, the language is much stricter. This makes it easier to parse, but it offers less leeway when writing it.
XHTML has its own set of versions (1.0, 1.1). HTML is still being worked on; there is a version 5.0 now (or nearing completion).