A computer generally means a programmable machine. The two principal characteristics of a computer are: it responds to a specific set of instructions in a well-defined manner and it can execute a prerecorded list of instructions (a program).
Modern Computers Defined
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:
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.
Computer Classification: By Size and Power
Computers can be generally classified by size and power as follows, though there is considerable overlap:
The First Computer
The first computer, the ENIAC machine (John W. Mauchly and J. Presper Eckert at the University of Pennsylvania) used 18,000 vacuum tubes and trequired 167 square meters (1800 square feet) of floor space (source).