X Windowing System, also known as X, takes a client-server architecture to handling graphics. It is the de-facto display protocol used by free UNIX-like operating systems.

It is officially being replaced by the newer Wayland protocol.

A simple X displaying system consists of a X server which communicates with applications which are X clients, using the X11 protocol. X extensions and libaries add on features to the basic server

e.g.: Xlib, XCB (X C Bindings), XKB (X Keyboard Library, XFreeType (FreeType font library)

Terminology

Xorg - an open-source implementation of X Windowing System display server
X11 - an X-Server-X-Client communication protocol

X Servers

X.Org is the main server used everywhere.
Others are:
XFree86 was the original server, that was replaced by X.Org.
XQuartz allows running X applications over Quartz
Xming is a free X server for Windows
Xvnc allows running X applications over VNC

Windowing Systems

Wikipedia Outline:

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