XSLT is a language for describing how an XML document is transformed into another. An XSLT stylesheet is made up of templates that match against the incoming tree and produces the result tree. XSLT is itself an XML document too.
XSLT is often used to produce presentational data from a pure content document. For example to create HTML (or XHTML) from a document consisting of made-up tags specific to an application.
XSLT is a standard being maintained by the W3 Consortium.
XSLT was formerly just a part of XSL.