Inherits from SimGroup, so that controls can have children.
Interfacing with a GuiControlProfile.
An abstraction from the details of handling user input and so forth, providing friendly hooks like onMouseEnter(), onMouseMove(), and onMouseLeave(), onKeyDown(), and so forth.
An abstraction from the details of rendering and resizing.
Helper functions to manipulate the mouse (mouseLock and mouseUnlock), and convert coordinates (localToGlobalCoord() and globalToLocalCoord()).
GUI has an overview of the GUI system.