The most important difference between the two markup languages is that HyperText Markup Language, or HTML, is an application of SGML (Standard Generalized Markup Language), and allows an author to omit certain tags and use attribute minimization.
The Extensible HyperText Markup Language, or XHTML, is an application of XML (Extensible Markup Language). It 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. A conforming XML document must be well formed, which, among other things, means that there must be an end tag for every start tag, and that nested tags must be closed in the right order.
For More Information You Can Read These Books Listed Below : -
1. HTML & XHTML: The Definitive Guide, Fifth Edition By
Chuck Musciano (Author), Bill Kennedy (Author)
2. HTML & XHTML: the complete reference
By Thomas A. Powell