Basically, XBL is a langauge that describes a set of bindings that can be attached and bound to elements on the screen. When bound, the binding implements new methods and properties to the associated element. In essence, the widgets on the screen are layed out using XUL and then given functionality using XBL.

::The XBL file used in developing XUL based applications is used to provide a specified set of functionality to individual elements that are rendered onto your screen.