What's the difference between XSLT and XSL-FO?

I was wondering about the basis of the question because I thought it was easy to answer, however as soon as you go here: http://www.w3.org/TR/xsl/ it becomes clear that its actually a good question - because pretty much the first thing on the page is this statement:

This specification defines the features and syntax for the Extensible Stylesheet Language (XSL), a language for expressing stylesheets. It consists of two parts:

1. a language for transforming XML documents (XSLT), and
2. an XML vocabulary for specifying formatting semantics.

However, back in the real world, XSLT (which is also what most people will assume you mean by XSL) is a means for transforming XML documents into something else - that something else more often than not being another well structured document probably also formatted as XML (though I've used XSLT to render XML to csv and plain(ish) text).

XSL-FO on the other hand is about formatting - about laying out content on a page or a sequence of pages in a fairly strict fashion. Its useful because it allows you to manage content that is spread across multiple pages, its relatively straightforward to specify the format of a page (or even and odd pages) including headers, footers, borders, columns, etc and have your content flow into that. One would therefore take a load of data in, say, an XML format and then use XSLT to convert that data into a document consisting of XSL-FO that in turn is rendered using an appropriate tool in say PDF for print or other means of distribution.

The use case I have is to take a pile of tabular data, export that data as XML, render that into XSL-FO and from there to PDF which goes to a printer who turns said PDF into a 500 page directory. One specifies in the XSL-FO that one wants page numbers, page breaks in specific circumstances, that there is a table of contents and an index (each based on particular elements) and the rendering process (to PDF) handles filling in the page numbers across the board.

Hopefully you're a bit less confused now..

XSLT is used for transforming, XSL (which is the same as XSL-FO) is used for formatting.

http://www.w3schools.com/xslfo/xslfo_intro.asp

What about the Wikipedia definition?

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, a set of W3C technologies designed for the transformation and formatting of XML data. The other parts of XSL are XSLT and XPath. As of December 12, 2006, the current version of XSL-FO is v1.1.

XSL-FO stands for Extensible Stylesheet Language Formatting Objects and is about formatting and now is known as XSL to avoid all confusion. So do not worry about XSL-FO anymore.

XSLT however is about transforming xML. So it is very clear. XSL is about formatting and XSLT is about transforming.

XSLT is a template format that can be used to generate all sorts of documents, including plain text documnent.

XSL-FO denotes the use of XSLT to produce FO documents.