X windows is not part of the operating system on any of the systems it runs on. Instead it is a user application built as an additional layer on top of whatever the host operating system is.
X uses a client-server model: an X server communicates with various client programs. The server accepts requests for graphical output (windows) and sends back user input (from keyboard, mouse, or touch screen). The server has function displaying to a window of another display system, a system program controlling the video output of a PC, a dedicated piece of hardware. And the function of clients includes: administering a remote machine graphically, running a computationally intensive simulation on a remote UNIX machine and displaying the results on a local Windows desktop machine.
The communication protocol between server and client operates network-transparently: the client and server may run on the same machine or on different ones, possibly with different architectures and operating systems, but they run the same in either case. A client and server can even communicate securely over the Internet by tunneling the connection over an encrypted network session.
There are many popular desktop environments and each user is free to choose which the best desktop environment for them. The result is that, if you use an all KDE or all, gnome system, the look and feel of the environment is very consistent and your applications all interact between them. This just wasn’t possible when we had application written in variety of different toolkits. The range of facilities provided by modern desktop environment under Linux also enable some other niceties, like component architecture which allow you to do thing like having a live spreadsheet or chart inside a word processing document, global printing facilities, similar to the printing contexts found in windows or scripting language, which let more advanced users write program to glue application together and have them interact and cooperate ways.
X window is a system (application) that running in certain Operating System for achieving Graphical User Interfaces (GUI). One reason X has had such staying power is that from the beginning it incorporated many of the windowing capabilities that we now take for granted. These capabilities include network transparency, graphical capability, the use of a mouse, and the ability to link together a heterogeneous network of workstations from different vendors.
One of the most significant features of X is that it was specifically designed to work across a network. The client and the server communicate via the X Protocol, a network protocol that can run locally or across a network. Regardless of whether a client program is local or remote, it communicates with the server through the X Protocol.
X window in windows operating system have function to build interface. as if we used windows operating system and want to used Mac interface we can used X window with other application help. So the function X windows in windows operating system have function just build interface, it don’t have function as GUI. Windows constant used their own GUI.