XSL-FO: XSL Formatting Objects
The other part of XSL is XSL-FO, which is the formatting language part of XSL. You can use XSL-FO to specify how the data in XML documents is to be presented, down to the margin sizes, fonts, alignments, header and footer size, and page width. When you're formatting an XML document, there are hundreds of items to think about, and accordingly, XSL-FO is much bigger than XSLT.
On the other hand, because of its complexity, XSL-FO is not very popular yet, certainly not compared to XSLT. There's not much software that supports XSL-FO at this point, and none that implements anywhere near the complete standard. Just as the most common use of XSLT is to transform XML to HTML, the most common use of XSL-FO is to convert XML to formatted PDF (Portable Data Format), the format used by Adobe Acrobat.