Extensible Stylesheet Language (XSL) is concerned about a family of languages:
1. XSL Transformations (XSLT)
2. XML Path Language (XPath)
3. XSL Formatting Objects (XSL-FO)
which are used to transform and render XML documents.
XSLT is a language for transforming XML documents.
XPath is an expression language which is used by XSLT to refer to parts of an XML document.
XSL-FO is an XML language for visual formatting of an XML document.
As we use CSS for HTML document to display the content in some specific color font etc. In the same way XSL can also be used as the style language which declares how to display content of an XML document.