The X Windows system provides the framework for a GUI environment – things like drawing and moving windows on the display and interacting with input devices like the mouse, the keyboard, and touchscreens. The protocol version has been X11 for over two decades, so it also known as X11.
The reference implementation for the X Windows system is the X.Org server, which is released under permissive licenses such as MIT. It uses a client/server model, with the server communicating with several client programs, serving user input, and accepting graphical output. The X11 protocol is network transparent so that the clients and the server may run on different machines, with different architectures and operating systems. However, mostly, they both run on the same machine and communicate using local sockets.
User interface specifications, such as buttons or menu styles, are not defined in X11, which leaves it to other window manager applications that are usually...