Conceptually a file is a named container in which a collection of data that belongs together can be stored. It might contain, for example, a program or input data for a program. Each file is given its own name so that it can be easily referred to. The term stream refers to any input source or output destination. Input can be from the key board, from secondary storage or from some other external device.
Output can be to the screen, to secondary storage or to some other external device.
An instruction tells the computer that it should perform a particular task. Examples included arithmetic instructions such as addition and subtraction, and logic instructions which evaluate to true or false. When a program is running in a computer, it is said to be executing. At this time a copy is contained in primary memory where it occupies a number of consecutive memory cells. On completion control is returned to the operating system.