XHTML is a markup language for Web pages from the W3C(World Wide Web Consortium). XHTML combines HTML and XML into a single format (HTML 4.0 and XML 1.0). Like XML, XHTML can be extended with proprietary tags. Also like XML, XHTML must be coded more rigorously than HTML.
Over the years, HTML coders have become sloppy, because Web browser software was originally written to tolerate many variations in HTML coding, but, with XHTML, coders must conform to the XML rules.
In one sentence we can say that XHTML is a superset of HTML, but unlike HTML it is stricter to rules and requires a document to follow XML rules.
Whereas HTML is an application of SGML, a very flexible markup language, XHTML is an application of XML, a more restrictive subset of SGML.
Because they need to be well-formed, true XHTML documents allow for automated processing to be performed using standard XML tools unlike HTML, which requires a relatively complex, lenient, and generally custom parser.
XHTML can be thought of as the intersection of HTML and XML in many respects, since it is a reformulation of HTML in XML.