XML elements are used to define WoW User Interface elements. These elements are defined using FrameXML '.xml' files which are loaded by WoW. When a FrameXML file is loaded, it processes the file's XML and from it's XML elements generates the actual game elements, which can be accessed programatically using the UIObjects in the WoW Lua environment. The root element of the WoW XML hierarcy is the Ui element. See XML UI for a general overview.