So what is XUL anyway? It is an XML language whose tags consist of:
(1) layout primitives (tags like hbox, vbox, grid and stack)
(2) widgets (tags like menulist, menubar, toolbar, and button)
(3) commands, keyboard accelerators (tags like command and keyset)
(4) xul templates (for UI binding to back-end data, represented as RDF)
Hyatt's 1-2 page description of Mozilla's XUL implementation is by far the best concise description I've seen anywhere. He understands this material, and he can communicate it. Quite a neat little artifact that he seems to have tossed out fairly quickly.