XSLT

XSLT is part of  XSL, the Extensible Stylesheet Language,  the  stylesheet language for XML.  XSL specifies the styling of an XML document by using XSLT to describe how the XML source document is transformed into another XML result document that is then formatted using the formatting vocabulary part of the XSL specification.

XSLT was designed to be used with an XSL formatter, or to stand alone. While XSLT can be used to do XML transformations, it was not intended as a completely general-purpose XML transformation language. Rather it was designed primarily for the kinds of transformations that are needed when we must transform text for formatting.

A transformation in the XSLT language is expressed as a well-formed XML document conforming to the Namespaces in XML Recommendation. It may include both elements that are defined by XSLT and elements that are not defined by XSLT. 

A transformation expressed in XSLT describes rules for transforming a source tree into a result tree. The transformation is achieved by associating patterns with templates. A pattern is matched against elements in the source tree. A template is instantiated to create part of the result tree. The result tree is separate from the source tree. The structure of the result tree can be completely different from the structure of the source tree. In constructing the result tree, elements from the source tree can be filtered and reordered, and arbitrary structure or data can be added.  A set of XSLT transformation rules is called a stylesheet whether or not formatting is applied.

XSLT 1.0 is a W3C Recommendation.  A Working Draft for XSLT 1.1 ws posted by the W3C in the summer of 2001.