The X Window System
The X Window System is a portable, multi-user graphical window system originally developed at MIT. "Portable" means that it can run on many types of host computers (including OpenVMS and a variety of Linux and UNIX hosts). "Multi-user" means that many users can have simultaneous access to X applications by connecting to the host over a network. As a window system, X allows users to run several applications at one time, each in its own window (similar to Microsoft Windows).
X applications (clients) provide the graphical user interface for many host-based environments — a few well-known environments are the GNOME Desktop Environment (GDE), the K Desktop Environment (KDE), and the Common Desktop Environment (CDE). X applications are also commonly developed for engineering, scientific, and manufacturing purposes.
The X Window System is based on a client/server model:
This division in the X Window System architecture allows the clients and the display server to reside on different types of computers. The client applications reside on UNIX hosts while the server runs on the local computer. With Reflection X Advantage this can be a Windows or UNIX computer.
Because the X display server runs on a local computer, client/server terminology often seems reversed to people new to the X Window System. As a display server, the local computer provides the services of the local display, keyboard, and mouse to applications (clients) running on other computers.
Requests, Replies, Events, and Errors
The X server and X client applications communicate by the following means: