Windowing system
A window system is a standard part of all modern computer graphical user interfaces, as opposed to command line interfaces. The idea was developed as a part of the WIMP paradigm at Xerox PARC.
A window system enables the computer user to work with several programs or applications at the same time. Each program runs in its own window, which is a rectangular area of the screen. Most window systems allow windows to overlap, and provide means for the user to perform standard operations such as moving/resizing a window, sending a window to the foreground/background and minimizing/maximizing a window.
From a programmer's point of view, a window system implements graphical primitives such as rendering fonts or drawing a line on the screen, effectively providing an abstraction of the graphics hardware.
Some window systems, like X, have advanced capabilities such as network transparency, allowing the user to run graphical applications on a remote machine.
See also
Specific GUI windows systems: