XHTML is not so much different from HTML 4.01 standard. The major differences are:
- XHTML elements must be properly nested.
- XHTML elements must always be closed.
- XHTML elements must be in lowercase.
- XHTML documents must have one root element.
Basically, XHTML is HTML (all the html tags are found in XHTML) that follows the rules of XML (because it is a family of XML).
More references can be found on:
- W3Schools.
- Wikipedia.
- Sitepoint.com.