A markup language similar to HTML and based on XML.

XUL used to define what the user interface will look like for a particular piece of software. XUL is used to define what buttons, scrollbars, text boxes, and other user-interface items will appear, but it is not used to define how those item will look (e.g. what color they are).

The most widely used example of XUL use is probably in theMozilla web browser, where the entire user interface is defined using the XUL language.