A windowing system is a system for sharing a computer's graphical display presentation resources among multiple applications at the same time. In a computer that has a graphical user interface (GUI), you may want to use a number of applications at the same time (this is called task). Using a separate window for each application, you can interact with each application and go from one application to another without having to reinitiate it. Having different information or activities in multiple windows may also make it easier for you to do your work.
A windowing system uses a window manager to keep track of where each window is located on the display screen and its size and status. A windowing system doesn't just manage the windows but also other forms of graphical user interface entities.
The X Window System is a cross-platform windowing system that uses the client/server model to distribute services in a network so that applications can run in a remote computer. Users of workstations or terminals using the X Window System don't need to know where the application is located. Apple's Macintosh and Microsoft's Windows operating systems have their own windowing systems built into the operating system.