GUI
From Hackepedia
Graphical User Interface
There are usually 3 levels of user interface..
Terminal: which is typically the white text on black background like on your first computer. If you're in X windows, you can see an x terminal by trying:
$ xterm
ncurses: Not really a gui, but a prettier way to use terminals. If you're in a terminal and have dialog installed:
$ dialog --yesno "Is this site helpful?" 7 60
and then the GUI, which is typically a window where you will use your mouse to point and click instead of typing.