While HTML is used for writing website, the XML user Interface
(pronounced "zool") is used for building applications.
XUL can be used to program interfaces for Mozilla projects but it
is also a platform independent programming language, which
sepearates content from presentation.
ExamplesYou can start an XUL page by opening it with Mozilla or firefox.
Simplicity, Clarity, Generality B.W. Kernighan, R. Pike, in "The Practice of Programming".