What is a Terminal ?

A terminal consists of a screen and keyboard that one uses to communicate remotely with a (host) computer. One uses it just like it was a personal computer but the terminal is remote from its host computer (on the other side of the room or even on the other side of the world). Programs execute on the host computer but the results display on the terminal screen. Its computational ability is relatively low (otherwise it would be a computer and not a terminal). This computational ability is generally limited to the ability to display what is sent to it (possibly including full-screen graphics) and the ability to send to the host what is typed at the keyboard.

In the days of mainframes from the mid 1970's to the mid 1980's, most people used text terminals to communicate with computers. They typed in programs, ran programs, wrote documents, issued printing commands, etc. A cable connected the terminal to the computer (often indirectly). It was called a terminal since it was located at the terminal end of this cable.

If you've been using Linux (except for X Window use) with a computer monitor and keyboard you already know what a terminal is because you have been using one (or more precisely a "virtual terminal"). The monitor (along with the keyboard) is emulating a terminal. In X Window the programs xterm, rxvt, and zterm emulate terminals.

A real terminal is different from a monitor because it's a different electronic setup. A text terminal is often connected to a serial port of the computer via a long cable. Thus, in contrast to a monitor which is usually located right next to the computer, a terminal may be quite a distance away from its host computer. The video card inside a computer stores the video image seen on the monitor screen. For a terminal, the equivalent of this video card is built right into the terminal but since text terminals are often monochrome without much graphics, the capabilities of its "video card" are rather weak. Also, most text terminals do not have mice.

In network client-server terminology, one might think that the terminal is the client and that the host computer is the server. The terminal has been called a "thin client" by some. But it is not actually a "client" nor is the host a "server". The only "service" the host provides is to receive every letter typed at the keyboard and react to this just like a computer would. The terminal is like a window into the computer just like a monitor (and keyboard) are. You may have already used virtual terminals in Linux (by pressing Left Alt-F2, etc.). A real terminal is just like running such a virtual terminal but you run it on its own terminal screen instead of having to share the monitor screen. In contrast to using a virtual terminal at the console (monitor), this allows another person to sit at the real terminal and use the computer simultaneously with others.