XUL applications are application built on top of the Mozilla Core
and Geck as the GUI rendering engine.
Firefox and Thunderbird for example are also XUL applications.
Creating your own XUL application is very simple : A simple XUL application
The next question should be : How do i run a XUL application ? Thanks to guys at Mozilla
there exists an relatively new application called XULRunner. XULRunner acts as the runtime engine
for XUL applications;
EclipseXUL supports development of XULRunner based applications by providing a special XULRunner
Nature, Perspective, Builder, Wizards and so on.