XSL, or Extensible Stylesheet Language, is a language used for transforming and rendering XML documents. It allows developers to define how the data in an XML file should be displayed or formatted, making it easier to present information in a user-friendly way. XSL consists of three parts: XSLT (XSL Transformations), which is used to transform XML data into other formats like HTML or text; XPath, which is a language for navigating through elements and attributes in an XML document; and XSL-FO, which is used for formatting output for print and other media.