X Window System Concepts

This is a good article about X: Xlib Programming Manual http://www.sbin.org/doc/Xlib/index_contents.html
A display is defined as a workstation consisting of a keyboard, a pointing device such as a mouse, and one or more screens.

Server-client modal

X is a network-oriented windowing system. An application need not be running on the same system that actually supports the display.

the program that controls each display is known as a server.

The server acts as an intermediary between user programs running on either the local or remote systems and the resources of the local system.

Window Management
Another important concept in X programming is that applications do not actually control such things as where a window appears or what size it is. Given multiprocessor, multiclient access to the same workstation display, clients must not be dependent on a particular window configuration. Instead, a client gives hints about how long and where it would like to be displayed. The screen layout or appearance and the style of user interaction with the system are left up to a separate program, called the window manager.