XUL is a language similar to HTML, which is used to create applications rather than web-pages. If you come from the Microsoft world, it's like XAML except that it exists and it works. Follow the link above for a summary of the technology from the people who created it
The XUL project page, at Mozilla
There is a tutorial to XUL, and a reference guide. (Beware links to xulplanet.com which aren't working at the moment)
There is a development environment for XUL, again at Mozilla
