Extensible Hyper Text Markup Language is an upgrade to Hyper Text Markup Language and is also future standards compliant.
XHTML is stricter than HTML, so you have to watch how you code your site. As XHTML is future proof though, it would be better to use it over HTML.
Did you ever see a
icon on some sites? This means they are XHTML compliant. You'll see it mainly on forums, content management systems and portals.
Hopefully that answered your questions.