XHTML has been introduced by W3C to gradually substitute plain HTML and prepare the web for a transition to a tidier, stricter code practice compatible with XML. XHTML is in effect a hybrid of XML and HTML, and while most websites are not serving it as XML yet (the correct MIME type is XML, put it can cause problems in some circumstances), serving it as as html/text still has a purpose while we wait for browsers and mobiles to adapt.
In general, it should be possible to convert an old HTML document to XHTML by following a few simple practices:
On this site we feature several cheat sheets and tutorials to help you transition to XHTML and understand it better, enjoy it!