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In computing, the X Window System (commonly X11 or X) is a networking and display protocol which provides windowing on bitmap displays. more...

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It provides the standard toolkit and protocol to build graphical user interfaces (GUIs) on Unix, Unix-like operating systems, and OpenVMS, and is supported by almost all other modern operating systems.

X provides the basic framework, or primitives, for building GUI environments: drawing and moving windows on the screen and interacting with a mouse and/or keyboard. X does not mandate the user interface – individual client programs handle this. As such, the visual styling of X-based environments varies greatly; different programs may present radically different interfaces.

X features network transparency: the machine where application programs (the client applications) run can differ from the user's local machine (the display server). X's usage of the terms "client" and "server" reverses what people often expect, in that "server" refers to the user's local display ("display server") rather than to a remote machine.

X originated at MIT in 1984. The current protocol version, X11, appeared in September 1987. The X.Org Foundation leads the X project, with the current reference implementation, version 11 release 7.1, available as free software under the MIT License and similar permissive licenses .

The X client-server model and network transparency

X uses a client-server model: an X server communicates with various client programs. The server accepts requests for graphical output (windows) and sends back user input (from keyboard, mouse, or touchscreen). The server may function as:

an application displaying to a window of another display system; a system program controlling the video output of a PC; a dedicated piece of hardware.;

This client-server terminology — the user's terminal as the "server", the remote applications as the "clients" — often confuses new X users, because the terms appear reversed. But X takes the perspective of the program, rather than that of the end-user or of the hardware: the local X display provides display services to programs, so it acts as a server; any remote program uses these services, thus it acts as a client.

The communication protocol between server and client operates network-transparently: the client and server may run on the same machine or on different ones, possibly with different architectures and operating systems, but they run the same in either case. A client and server can even communicate securely over the Internet by tunneling the connection over an encrypted network session.

To start a remote client program displaying to a local server, the user will typically open a terminal window and telnet or ssh to the remote machine, tell it to display to the user's machine (e.g. export DISPLAY=:0 on a remote machine running bash), then start the client. The client will then connect to the local server and the remote application will display to the local screen and accept input from the local input devices. Alternatively, the local machine may run a small helper program to connect to a remote machine and start the desired client application there.

Read more at Wikipedia.org