XHTML stands for "Extensible Hypertext Markup Language."
XHTML is a markup language (like HTML) that combines features of both HTML and XML. It was developed as a version of HTML based on XML rather than SGML, since XML is a much simpler and, thus, more flexible markup language than SGML. XML lets you introduce or add new elements easily more easily than SGML.
XHTML was created by revising HTML to make it compatible with XML, so that documents written in XHTML would adhere to the rules of XML.
HTML is much less strict in its tagging practices than XML. A browser will, for instance, display HTML code even if some of the tags do not close, while XML-based documents have to be "well-formed" in order to be displayed by a browser. In general, a document written in XHTML is much cleaner than one written in HTML.
XHTML is recommended for content development by the W3 Consortium.