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A computer is a device that processes data according to a set of instructions called a program. Most modern computers are electronic and digital. The machinery of computers, such as the circuitry and wires, is known as hardware; the instructions and data are called software.

All general-purpose computers have the following hardware components:
  • memory : Enables a computer to store, at least temporarily, data and programs
  • mass storage device : Allows a computer to permanently retain large amounts of data. Common mass storage devices include disk drives and CD-ROMs
  • input device : Usually a keyboard and mouse, the input device is the conduit through which data and instructions enter a computer
  • output device : A display screen, printer, or other device that lets you see what the computer has accomplished
  • central processing unit (CPU): The heart of the computer, this is the component that actually executes instructions
Expert predictions that went wrong
"I think there's a world market for maybe five computers. "
    – Thomas Watson, chairman of IBM (1943)

"There is no reason why anyone would want to have a computer in their home. "
    – Ken Olsen, president of Digital Equipment Corporation (1977)

"640 K [of computer memory] ought be enough for anybody."
    – Bill Gates (1981)
In addition to these components, many others make it possible for the basic components to work together efficiently. For example, every computer requires a bus that transmits data from one part of the computer to another.

Computers can be generally classified by size and power as follows, though there is considerable overlap:
  • personal computer : A small, single-user computer based on a microprocessor. In addition to the microprocessor, a personal computer has a keyboard for entering data, a monitor for displaying information, and a storage device for saving data
  • workstation : A powerful, single-user computer. A workstation is like a personal computer, but it has a more powerful microprocessor and a higher-quality monitor
  • minicomputer : A multi-user computer capable of supporting from 10 to hundreds of users simultaneously
  • mainframe : A powerful multi-user computer capable of supporting many hundreds or thousands of users simultaneously
  • supercomputer : An extremely fast computer that can perform hundreds of millions of instructions per second

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