XSL Formatting Objects (XSL-FO) is a markup language for XML document formatting which is most often used to generate PDFs.
The general idea is that user writes a document, not in FO, but in an XML languag like XHTML, DocBook or any other XML language. Then, an XSLT transform converts the XML into the resulting document.
XSL-FO is part of XSL, a set of W3C technologies designed for the transformation and formatting of XML data.
