The meanings of these two are different. XHTML stands for Extensible Hypertext Markup Language while HTML almost means the same but minus the first word. These are programming languages that programmers work with whenever they need to create different web pages and applications.
XHTML stands for Extensible Hypertext Markup Language and HTML is just Hypertext Markup Language. Both of these are types of programming languages. An XML syntax is used with XHTML, while a psuedo -SGML syntax is used with HTML. Content that requires namespaces will work with XHTML, but not with HTML.