"Learning XML for formatting semantics"
XSL-FO stands for Extensible Stylesheet Language Formatting Objects. XSL-FO documents are expressed in XML-format, and hence follow common XML-syntactical rules.
This technology can be used in many areas, for instance generating print or display outputs, or even many other multimedia documents, including audio. The most common example though, is to generate a pdf-file from a xml source. Other examples could be to take an XML-file containing some data, and convert it to a formatted email or letter, ready to be sent.
XSL-FO is well suited to integrate with XSLT and XPath.