XSLT (XSL Transformations, Extensible Stylesheet Language Transformations) is a programming language that is based on XML and whose purpose is to manipulate and transform XML. It is a functional programming language and a W3C standard. Using XSLT you can transform one or more XML documents into a result that could have variety of output formats (XML, HTML, text).

To understand XSLT -- it is best to have a good working knowledge of:

1. XML
2. XPath

Without these two underlying concepts you would have difficulty learning XSLT. Firstly, because an XSLT document is an XML document. And secondly, XSLT documents make use of XPath to query the XML document being transformed.