XSLT stands for the eXtensible Stylesheet Language Transformations. It is part of the XSL specification which includes the transformation and formatting objects (XSL-FO). XSL Tools primarily deals with the editing, launching, and debugging of XSL Style sheets.
Getting Started
Rescuing XSLT From Niche Status - gives a basic introduction to XSLT from the point of view of creating HTML output.
XSLT and XPath Quick Reference Cards - a set of PDF files that can be printed to provide quick references to the XSLT and XPath commands.