The X window system (commonly X Window System or X11, based on its current major version being 11) is a display server and network protocol that provides a basis for a GUIIndex graphical user interfaces (GUIs) and rich input device capability for networked computers. In Liunux it sits between the Operating System kernel and the WindowManagers Window Manger.

X is primarily a protocol and graphics primitives definition and it deliberately contains no specification for application user interface design, such as button, menu, or window title bar styles.

It has been commented that X is outdated. Two proposed successors are Wayland and Mir.

Most common implementation of the X Window System X.Org

X.Org libraries: http://xorg.freedesktop.org/releases/individual/lib/?C=M;O=D

Video information: /var/log/Xorg.0.log

Use X -version to display version information.

Xlib - functions to interact with X server

WikiPedia:X_Window_System - Overview of protocol

WikiPedia:X_Window_System_core_protocol - Details of core protocol

WikiPedia:Xlib - X Window System protocol client library

State of Xorg Report 2012

The Real Story Behind Wayland and X - Daniel Stone (youtube)