XUL which is pronounced zool stands for <b>X</b>ML <b>U</b>ser Interface <b>L</b>anguage is a Mark-up language used primarily in Mozilla applications. It allows for XUL is used to define what the creation of feature rich visually appealing cross platform applications user interface will look like such as buttons and other widgets but it is not used to be easily created and maintained for online or offline usedefine how those item will look.

XUL is an [http://en.wikipedia.org/wiki/Xml XML] language and thus has the advantages of being cross-platform also allows for the insertion of other XML languages into it such as [http://en.wikipedia.org/wiki/Xhtml XHTML], [http://en.wikipedia.org/wiki/MathML MathML] and [http://en.wikipedia.org/wiki/Svg SVG]. Also because of its localized structure it can be easily translated into other languages. XUL is often used for creating Mozilla extensions, such as [http://chatzilla.hacksrus.com/ ChatZilla]. Web applications can and do utilize XUL, but these are rare (typically proof-of-concept, such as the [http://faser.net/mab/chrome/content/mab.xul Mozilla Amazon Browser]). These XUL-based web applications can be confused for a native desktop application as they share many similar graphical widgets (text box, button, label, list box, check box, tree, menu, toolbar and more). Tip: If a web application is the goal, it is wise to use XUL with the XMLHttpRequest JavaScript object to make a responsive, "snappy" application that not only looks like a desktop application, but acts likes one speed-wise. Standalone XUL applications are more popular than web-based ones as the browser restricts operations (due to security concerns). When developers seek to add more functionality, a XUL application can be installed locally and will utilize [[Chrome]] so that it may be granted elevated privileges.