XSL Formatting Objects, or XSL-FO, is a markup language for XML document formatting which is most often used to generate PDFs. XSL-FO is part of XSL (Extensible Stylesheet Language), a set of W3C technologies designed for the transformation and formatting of XML data. (From Wikipedia)

Some useful Links

G. Ken Holman: What is XSL-FO, by
Antenna House: What is XSL Specification
w3c: The Extensible Stylesheet Language Family (XSL)
Wikipedia: XSL Formatting Objects