X Windows provides a standard toolkit and protocol for creating a Graphic User Interface on a graphics display. Typically the display is a bitmap graphics adapter. By using this toolkit, a developer can easily develop a windowed desktop with a mouse pointer interface. The platform has been ported to most major platforms and operating systems.
Because X Windows is simply a framework, it leaves a lot of the implementation of the desktop up to the user. In fact, because the framework is so flexible, it is very common for a desktop system to allow the user to "skin" or apply a theme to the desktop. Nearly every aspect of the windows system can be customized. Windows borders, title bars, inputs (like buttons, checkboxes and text inputs) and everything else to do with the user interface can be customized.
The first version of X was developed in 1984 by MIT. A new, more advanced version called X11 was released in 1987. The X.Org foundation now leads project development and the most current version is X11 release 7.1. It is published under the MIT license and is classified as free software since it has a very permissive license.