The term mainframe has shifted from its original reference to the main housing, or frame, that contained the central processing unit (CPU) of the computer. In those days, all computers were big-like the size of a garage-and the frame for the CPU might have been as big as a walk-in closet. Now mainframe refers to the kind of large computer that runs an entire corporation.
While "large" can still mean as big as a room, most of today's mainframes are much smaller, although they're still quite a bit bigger than a personal computer or even a minicomputer. A mainframe has an enormous storage space on disk and tape (like thousands of kilobytes, measured in gigabytes), and an enormous amount of main memory. Theoretically, it works a lot faster than the fastest personal computer. A mainframe also costs big bucks, from half a million or so on up.
Mainframes are tended by special technicians who feed them the programs they run and who scramble around trying to fix them whenever they stop working, which is often. All mainframes are multi-tasking, multi-user machines, meaning they are designed so many different people can work on many different problems, all at the same time.
Mainframes serve most often as information stores and processors. An army of smaller computers is connected to the mainframe. These smaller computers are not in the same room; they may be connected through phone lines across the world. Ordinary people in the company never touch the mainframe itself. Instead, they interact with the computer using a terminal, which is more or less a keyboard and a monitor connected to the mainframe with wires, or by modem over the phone lines. People use the smaller computers and get information from and send information to the mainframe.
The difference between a minicomputer and a mainframe is arbitrary, and different people may use either term for the same machine. Even if you don't work for a large company, you might have contact with a mainframe: when you connect to an online information service or a commercial e-mail service from your personal computer, you are often connecting to a mainframe.
In the '60s the mainframe vendors were called "IBM and the seven dwarfs": Burroughs, Univac, NCR, Control Data, Honeywell, GE, and RCA. They turned into IBM and the BUNCH after Honeywell ate GE's computer division and Univac ate RCA's.