XSL Formatting Objects (XSL-FO) are the second half of the Extensible Stylesheet Language (XSL). XSL-FO is an XML application that describes how pages will look when presented to a reader. A style sheet uses the XSL transformation language to transform an XML document in a semantic vocabulary into a new XML document that uses the XSL-FO presentational vocabulary.
From : http://www.cafeconleche.org/books/bible2/chapters/ch18.html