One of the earliest protocols to be used regularly is called telnet. The telnet protocol allows a user to access a remote computer. If you have the permission of the owner, you can use your computer as a terminal of another computer. If Terrence has an account on a big unix machine at work, and that computer and his home computer are both attached to the Internet, he can use telnet to have his computer 'call up' the unix machine at work. Once he does this, his computer is acting simply as a terminal. The screen he sees is being sent by the remote computer, and anything he types will be sent there. His own CPU is doing very little work. Any instructions he sends will go to the remote computer.
Why would anyone want to do this?
Telnet allows people to work on computers regardless of geographical limitations. It allows people to set up complex programs on mainframes or other computers, and let people log in to this computer. One good example is the library of Congress Information System (LOCIS). The Library of congress maintains a huge database. Through telnet connections, people can access the LOCIS computer from anywhere in the world.
Telnet is a very inexpensive protocol to implement and utilize. It does have disadvantages, however. The remote computer does all the work. It doesn't matter how nice the graphics capabilities are on your local computer, for example, because you will be forced to use plain text most of the time when working on a telnet connection. Also, not all computers on the Internet allow unlimited telnet access. (In fact, very few do.) It is far more likely that a computer will allow you access only if you already have an account on that machine. In other words, you must have previously arranged permission from the owner of the remote computer in order to use that computer. Telnet is still used extensively by computing professionals, and it is often used by large institutions such as libraries, universities, and government institutions as a very inexpensive way to provide access to information on mainframes. The Indiana University system, for example, uses a system called IUIS (IU Information System) for student scheduling and other purposes. If you have registered for class by computer at an IU campus, you probably used IUIS via telnet (!)
There are special programs designed specifically to help you use Telnet. These programs are generally referred to as terminal programs or telnet clients.