Apache FOP (Formatting Objects Processor) is the most popular XSL-FO implementation. It is used to create documents in PDF, PostScript and other formats.
XSL (eXtensible Stylesheet Language) is a W3C standard which consists of two parts:
Apache FOP currently supports a large subset of XSL-FO 1.0 and some of the new features to be introduced with XSL-FO 1.1. Details on the compliance levels can be found on the FOP website.
The software is written in Java and published under a liberal license, the Apache License version 2.0, which allows commercial use and redistribution. For details, please read the license text and the license FAQ on the ASF website.