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). There are many different kinds.

 

 
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.