OperatingSystems provide a consistent abstraction for underlying hardware.
WindowingSystems provide abstractions for the visible parts: Screens, keyboards, pointing devices, etc.
Only a small number of
WindowingSystems are in common usage.
The Amiga had one of the best early ones, had a great and loyal following, but never gained enough market share.
ApolloComputers had a nice
GraphicalUserInterface on Unix before the
XwindowProtocol took the stranglehold of the unix environment.
SunMicrosystems had its own NeWS (see
NetworkExtensibleWindowSystem) before X took over.
The
XwindowSystem includes an
XwindowServer to control the display which uses the
XwindowProtocol to talk to a variety of clients and
WindowManagers -- i.e. twm, mwm, fvwm, ... This stack runs on top of a variety of internet enabled
OperatingSystems, especially
UnixOperatingSystem and
VmsOperatingSystem. Servers and clients exist for the Apple and Microsoft environments also. Xorg is probably the most commonly used Xserver in use today. X has a well defined layered architecture that sits neatly above many
OperatingSystems.
Apple's
OperatingSystem has been essentially inseparable from its type of
WindowingSystems since the introduction of the Macintosh computer in the 1980's. Apple's
MacOsx windowing system (Quartz for the accelerated graphics, Aqua for the look-and-feel) descends from the
WindowingSystems of the NeXT computers (
DisplayPostscript); now the underlying model is based on PDF.
Microsoft has been working on Windows, its example of
WindowingSystems for several years now. Until NT, it always seemed to be grafted on top of it's
OperatingSystem, now they seem better integrated. This is the windowing system used on most personal computers these days.
PlanNineFromBellLabs had
EightAndaHalf 8&1/2; and now
rio which use the intrinsic protocols for file access: Styx.
InfernoOs uses the same protocol, but has a different windowing system -- the fact that both were created by the same hands is quite evident in their interoperability.