The Extensible HyperText Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML, but a stricter syntax. 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, XHTML documents allow for automated processing to be performed using a standard XML library—unlike HTML, which requires a relatively complex, lenient, and generally custom parser (though an SGML parser library could possibly be used). XHTML can be thought of as the intersection of HTML and XML in many respects, since it is a reformulation of HTML in XML. XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C recommendation May 31, 2001.

Source: webopedia.com

Web Development Services by Originet IBS Inc.

The Extensible Markup Language (XML) is a W3C-recommended general-purpose markup language that supports a wide variety of applications. XML languages or 'dialects' are easy to design and to process. XML is also designed to be reasonably human-legible, and to this end, terseness was not considered essential in its structure. XML is a simplified subset of Standard Generalized Markup Language (SGML). Its primary purpose is to facilitate the sharing of data across different information systems, particularly systems connected via the Internet[1]. Formally defined languages based on XML (such as RSS, MathML, GraphML, XHTML, Scalable Vector Graphics, MusicXML and thousands of other examples) allow diverse software to reliably understand information formatted and passed in these languages.
 

Source: wikipedia.org