X Window Programming/Introduction
This book is designed for people interested in learning how to program the X Window System and assumes some basic knowledge on how to use a computer, how to setup and use The X Window System, how to use a compiler, and how to write a program in the C programming language. This book is open work, so if you find any problems with terms or concepts, you can contribute an explanation to it. Feel free to participate; you are welcome to do so.
The X Window System (or X and X11 for short) is a server application that provides basic functions for displaying graphics and gathering user input transparently, either locally or over a network. X distributes user input to and accepts output requests from applications through the X Window System protocol.
X does not mandate the visual appearance and interface of applications, choosing instead to allow applications to use whatever visual style and interface is desired. As a result, different applications may present radically different interfaces.