The document discusses Extensible Stylesheet Language (XSL), which consists of three languages - XSLT for transforming XML documents, XPath for selecting parts of an XML document, and XSL-FO for formatting. It provides examples of using XPath to select elements from an XML document and XSLT templates to transform the XML and output HTML. Key elements like <xsl:value-of>, <xsl:for-each>, <xsl:if>, and <xsl:choose> are described for extracting data from XML and conditionally processing elements.