A programmable machine. The two principal characteristics of a computer are: It responds to a specific set of instructions in a well-defined manner. It can execute a prerecorded list of instructions (a program). Modern computers are electronic and digital. The actual machinery -- wires, transistors, and circuits -- is called hardware; the instructions and data are called software.All general-purpose computers require 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 tape drives.
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.
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.