The Webmaster's Lexicon: X

XHTML	Extensible HyperText Markup Language (XHTML) is a reformulation of HTML 4.0 as an application of XML 1.0. XHTML 1.0 specifies three DTDs that correspond to the HTML 4.0 DTDs, along with an XML namespace which is identified by a unique URI. XHTML 1.0 is promised to be the basis for a family of future document types which extend and subset HTML.
XML	Extensible Markup Language (XML) is an ISO compliant subset of SGML A metalanguage, it allows for custom tags to be processed. Custom tags will enable the definition, transmission and interpretation of data structures between organizations.
XSL	(Extensible Style Language) (XSL) is a stylesheet language designed for the Web community. It provides functionality beyond CSS (e.g. element reordering). XSL will be used where more powerful formatting capabilities are required or for formatting highly structured information such as XML structured data or XML documents that contain structrued data.