HTML is a markup language used for displaying text and documents across different platforms and machines. It was originally intended for a scientists to better display their data in tables, and has expanded to include hypertext, multimedia, as well as the style of the documents displayed. It allows an author to omit certain tags and use attribute minimization.
XHTML is an extensible markup language that was developed to retain the flexibility and power of HTML while reducing most of the complexity. It has tighter rules than HTML and allows for more ease in design from CSS (cascading style sheets) and other current languages. xhtml doesn’t permit the omission of any tags or the use of attribute minimization. However, it provides a shorthand notation for empty elements—for example, we could use <br/> instead of <br></br>—which HTML does not.
Here is a good site for tutorials and manuals:
http://search.oreilly.com/?q=xhtml