Each graphics window represents a particular application, such as a text editor, a debugger, an electronic mail manager, or a terminal emulator. X lets you open multiple windows and run several applications simultaneously. The activity within one window executes independently of the activity in the other windows. First, and foremost, however, X is a windowing system, not a complete operating system. UNIX is an operating system.
X mediates the communication between individuals and a computer network.
X creates windows that represent applications, but does not execute the applications itself. You can tailor X to suit your needs. For example, you can tell X to run certain applications automatically each time you start an X session. That saves you the trouble of starting them manually every time you log in.