X is based on a client/server model in which the X server is a program that runs on your system and handles all access to the graphics hardware. An X client is an applications program that communicates with the server, sending it requests, such as "draw a line" or "pay attention to keyboard input." The X server takes care of servicing these requests by drawing a line on the display or sending user input (via the keyboard, mouse, or whatever) to the client application. Examples of X clients are the now-famous image manipulation program GIMP and the many programs coming out of the aforementioned desktop environments KDE and GNOMEfor example, the KDE email program KMail.
It is important to note that X is a network-oriented graphics system. That is, X clients can run either locally (on the same system that the server is running) or remotely (on a system somewhere on a TCP/IP network). The X server listens to both local and remote network sockets for requests from clients. This feature is obviously quite powerful. If you have a connection to a TCP/IP network, you can log in to another system over the network and run an X application there, directing it to display on your local X server.
Further advantages of X are security (if the user so desires), modular separation of functions, and support for many different architectures. All this makes the X Window System technically superior by far to all other window systems.
The X Window System makes a distinction between application behavior and window management . Clients running under X are displayed within one or more windows on your screen. However, how windows are manipulated (placed on the display, resized, and so forth) and how they are decorated (the appearance of the window frames) are not controlled by the X server. Instead, such things are handled by another X client called a window manager that runs concurrently with the other X clients. Your choice of window manager will decide to some extent how X as a whole looks and feels. Most window managers are utterly flexible and configurable; the user can select the look of the window decoration, the focus policy, the meaning of the mouse buttons when the mouse cursor is on the background part of the screen rather than on an application window, and many other things by editing the configuration files of the window manager. More modern systems even let you configure those aspects over a GUI.
To fully understand the concept of window managers, you need to know that the window manager does not affect what the client application does within the window. The window manager is only in charge of painting the window decorationthat is, the frame and the buttons that let you close, move, and resize windows.
There can be only one window manager on any X server. Theoretically, it is even possible to completely do away with window managers, but then you would not be able to move windows around the screen; put a hidden window on top; or minimize, maximize, or resize windows unless the programs themselves provided this functionality.
Let's shortly mention the desktop environments again. A desktop environment such as KDE or GNOME is a collection of applications and tools with a common look and feel as well as many other common propertiesfor example, the menus of the applications could all be set up according to the same concepts. Desktop environments on X always need a window manager, as described earlier. Some desktop environments provide their own window manager (such as KWin in the KDE desktop environment), whereas others do not have their own window manager. It is up to the user to install a window manager of his or her choice.