XHTML is to XML as HTML is to SGML. That is, XHTML is a markup language that is similar to HTML, but with a stricter syntax. Like vanilla HTML, XHTML is an international standard whose specification is maintained by the World Wide Web Consortium. The current version of the specification is XHTML 1.1.
As the Mozilla Developer Center continues to grow and expand, more content will be availible at this address. Until such time, Wikipedia's XHTML article is a fine starting place for budding web developers seeking more information.