The full form of XSL is EXtensible Stylesheet Language

XSL (Extensible Stylesheet Language), formerly called Extensible Style Language, is a language for creating a style sheet that describes how data sent over the Web using the Extensible Markup Language (XML) is to be presented to the user. For example, in an XML page that describes the characteristics of one or more automobiles for an insurance company, a set of open and close tags might contain the name of an auto manufacturer. Using XSL, you could tell the Web browser that the auto manufacturer name should be displayed, where to display it on a page, and that it should be displayed in a bold font.Think of an XML page as similar to an HTML page (like the one you are reading now), but containing data in identified fields rather than text and graphics. XSL gives a developer the tools to describe exactly which data fields in an XML file to display and exactly where and how to display them. Like any style sheet language, XSL can be used to create a style definition for one XML document or reused for many other XML documents.

XSL is based on and extends the Document Style Semantics and Specification Language (DSSSL) and the Cascading Style Sheet, level 1 (CSS1) standards. XSL is developed under the auspices of the World Wide Web Consortium (W3C).