Full form of GUI is Graphical User Interface.

GUIs are visual (graphical) interfaces in which instead of typing in commands, the user can use a mouse to point and click objects on the screen.

The main features of GUI are Windows, Icons, Menus and Pointers, which make these Operating Systems interactive in nature. These provide GUI interface to facilitate move and straightforward interaction to the user.

Windows, MAC operating systems are examples of such GUI systems.

The GUI operating systems that provide graphical interface and allow access to user via mouse or track-pad / track-ball are often abbreviated to WIMP (Windows, Icons, Menus and Pointers). WIMP operating systems require much more RAM compared to command line OSs.