A windowing system (or window system) is a graphical user interface (GUI) which implements windows as one of its primary metaphors. It is normally one part of a larger desktop environment.
From a programmer's point of view, a windowing system implements graphical primitives such as rendering fonts or drawing a line on the screen, effectively providing an abstraction of the graphics hardware.
A windowing system enables the computer user to work with several programs at the same time. Each program runs in its own window, which is an area of the screen, typically a rectangle. Most windowing 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.
Some windowing systems, like the X Window System, have advanced capabilities such as network transparency, allowing the user to display graphical applications running on a remote machine. Further, the X Window System does not implement any specific policy regarding the look and feel of the graphical user interfaces, leaving that to the X window managers, widget toolkits and desktop environments.
Reference:
http://en.wikipedia.org/wiki/Windowing_system
