FOP is the world's first print formatter driven by XSL formatting objects and the world's first output independent formatter. It is a Java application that reads a formatting object tree and then renders the resulting pages to a specified output. Output formats currently supported are PDF, PCL, SVG, XML (area tree representation), Print, AWT and TXT. The primary output target is PDF.
FOP is part of Apache's XML project. The homepage of FOP is http://xml.apache.org/fop. Here you can find information about using and developing with FOP.
The goals of the Apache XML FOP Project are to deliver an XSL FO->PDF formatter that is compliant to at least the Basic conformance level described in the W3C Candidate Recommendation 21 November 2000, and that complies with the 11 March 1999 Portable Document Format Specification (Version 1.3) from Adobe Systems.
Conformance to the XML 1.0 Recommendation, XSLT 1.0 Recommendation and the XML Namespaces Recommendation is understood. Other relevant documents, such as the XPath and XLink Working Drafts, are referenced as necessary. The FOP Project will attempt to use the latest version of evolving specifications.
To reach this aim currently the layout system is being redesigned to better handle the formatting of all different types of formatting objects.
Information taken from http://xml.apache.org/fop
information accurate as of 11/21/2001