X Window System

X originated at MIT in 1984. The current protocol version, X11, appeared in September 1987.

X Window System (commonly X11 or X) is not an integral part of the operating system; instead, it is built as an additional application layer on top of the operating system kernel.

Unlike previous display protocols, X was specifically designed to be used over network connections rather than on an integral or attached display device. X features network transparency: the machine where an application program (the client application) runs can differ from the user's local machine (the display server).

This client-server terminology — the user's terminal as the "server", the remote or local applications as the "clients" — often confuses new X users, because the terms appear reversed. But X takes the perspective of the program, rather than that of the end-user or of the hardware: the local X display provides display services to programs, so it acts as a server; any remote program uses these services, thus it acts as a client.

What is the X Window System?

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The X Window System, or X, is a network-transparent window system. With X, multiple applications can run simultaneously in windows, generating text and graphics in monochrome or color on a bitmap display. Network transparency means that application programs can run on machines scattered through the network.

What is an X Server?

An X Server is a program that provides display and user input services to other programs. In comparison, a file server provides other programs with access to file storage devices. File servers are typically located in a remote location and you use the services of a file server from the machine that you are located at. In contrast, an X Server is typically running on the machine that you are located at; display and user input services may be requested by programs running on your machine, as well as by programs running on remote machines.

What is an X client?

An X client is a program that utilizes the display and user input services provided by an X Server. X clients may run on the same or disparate machine as the X Server that is providing display and user input services.

ssh -X qiang@129.49.32.81

You will need to have the X-server running on your local computer and then use ssh -X option to connect to the remote server.

Reference:

http://www.linux-tip.net/cms/content/view/302/26/

http://en.wikipedia.org/wiki/X_Window_System

http://x.cygwin.com/docs/faq/cygwin-x-faq.html#q-what-is-cygwin-x