X-Files

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

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It provides the standard toolkit and protocol with which to build graphical user interfaces (GUIs) on most Unix-like operating systems and OpenVMS, and has been ported to all other contemporary general purpose 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 is not an integral part of the operating system; instead, it is built as an additional application layer on top of the operating system kernel.

Unlike previous display protocols, X was specifically designed to be used over network connections rather than on an integral or attached display device. X features network transparency: the machine where an application program (the client application) runs can differ from the user's local machine (the display server).

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 7.3 (September 6 2007), available as free software under the MIT License and similar permissive licenses.

Design

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 or local 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.

Read more at Wikipedia.org