Extensible Stylesheet Language Transformations (XSLT) is an XML-based language used, in conjunction with specialized processing software, for the transformation of XML documents. Although the process is referred to as "transformation," the original document is not changed; rather, a new XML document is created based on the content of an existing document. Then, the new document may be serialized (output) by the processor in standard XML syntax or in another format, such as HTML or plain text. XSLT is most often used to convert data between different XML schemas or to convert XML data into web pages or PDF documents.
This description of XSLT is obtained from the Wikipedia XSLT article.