XSL stands for Extensible Style Language and is a family of transformation languages for expressing style sheets. XSL is the second style specification to be offered by the World Wide Web Consortium(W3C) after Cascading Style Sheets(CSS).

An XSL style sheet is a file that specifies how to display an XML document of a given type. XSL family has three languages:

• XSLT: XSL Transformation which is a transformation language for XML documents. XSLT is widely used for general purpose XML processing and generating HTML web pages from XML data.
• XSL-FO: XSL Formatting Objects, a language for expressing visual styles of webpages.
• XPath: XML Path Language, a non-XML language used by XSLT for addressing the parts of an XML document.

Read more about XSL on Wikipedia here.