XSL-FO makes XML documents flexible and multipurpose
XSL-FO enables you to format an XML document in multiple ways simultaneously, for example, a web page and a prepress layout.
XSL-FO is an extension of XSL that allows you to tweak the presentation of an XML document in multiple ways simultaneously, such as building both a web page and a prepress layout. You can use it as a tool to help build a central document library, and integrate it with a CMS for easy maintenance.
To use XSL-FO, you need a FOP. The apache project provides one for free.
(from the builder.com XML and Web Services E-Newsletter for April 19, 2004)