XUL (pronounced “zool”) is Mozilla’s XML-based user interface language that lets you build feature rich cross-platform applications that can run connected to or disconnected from the Internet. These applications are easily customized with alternative text, graphics, and layout so they can be readily branded or localized for various markets. Web developers already familiar with Dynamic HTML (DHTML) will learn XUL quickly and can start building applications right away.
Combined it with XPCOM/XPConnect, which are complementary technologies that enable the integration of external libraries with XUL applications, it provides a very powerful and flexible development platform.
Together, XPCOM and XPConnect enable developers to create XUL applications that require the raw processing power of compiled languages (C/C++) or access to the underlying operating system.