Differences Between XHTML, HTML, and XML

XHTML is HTML 4.0 Written in XML Format
On January 26, 2000, the W3C announced that HTML 4 has been reformulated into XHTML 1.0. XHTML is HTML 4 written as an XML application.

There are two primary parts to XHTML:

HTML 4
HTML 4 is a markup language used for displaying text and documents across different platforms and machines. It was originally intended for a very specific audience, and has expanded to include hypertext, multimedia, as well as the style of the documents displayed.

XML
XML is an extensible markup language that was developed to retain the flexibility and power of HTML while reducing most of the complexity.

XHTML combines the flexibility of HTML with the extensibility of XML.