From LQWiki
A computer program is a sequence of instructions that is understandable by a computer. Generally, it indicates which operations the computer should perform on a set of data. Usually it will be stored for reuse in one or many files. Complex programs (or applications) may be made from many smaller programs working together, though unix philosophy generally favours the adoption of small, terse programs that are responsible for fairly specific tasks.
Many thousands of programs may exist on a computer, and many programs may run at the same time. If the computer hardware and OS are capable of multi-tasking, and the program in question is sufficiently carefully written, many instances of one program may run at one time. Computer operating systems themselves constitute a collection of programs bundled together by the manufacturer, distributor or administrator.
The source code for programs is written in one or more programming languages. Some programs need to be compiled (or in the case of assembly language, assembled) before they can be interpreted by a processor. Those which do not need to be compiled (because they are interpreted by processes already running on the computer) are referred to as scripts.
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