You want to create simple GUI cross platform application without knowing the high level programming languages like C++, Java, C#~(using mono)? XUL is that answer.
XUL is basic building block of the Firefox UI and as we know Firefox works in Windows, Linux and MAC.
Actually XUL is a language which run on the XULRunner(Gecko) runtime from Mozilla.
If you really wants see how Firefox created this UI, go the FireFox installation folder. Normally it will be
C:\Program Files\Mozilla Firefox\. Under that installation root folder, find a folder named “chrome”. Firefox has packaged the xul files in a zip format and they have renamed it as
.jar. So look for a file
browser.jar and rename it to zip. Extract it and have a look at
content\browser\browser.xul. This file holds the UI definition for Firefox.
To learn more about XUL, refer these web sites