Also see file format.
1) A format (noun, pronounced FOHR-mat) is a preestablished layout for data. A computer program accepts data as input in a certain format, processes it, and provides it as output in the same or another format. All data is stored in some format with the expectation that it will be processed by a program that knows how to handle that format. Generically, data formats tend to fall into bitmaps (strings of 0s and 1s) that describe images or sound patterns (or both), text formats (in which usually each byte value is mapped to a character), and numeric data formats (used by spreadsheet and other database programs).
A hard disk or other storage device is also said to be formatted when its space has been organized and divided into pieces that can be controlled for convenient storage and access. For example, a hard disk may be formatted into areas called sectors, tracks, and clusters.
2) To format (verb) a document or file for printing or displaying is to add the necessary information to it so that the output device will know how to present the output.
3) To format (verb) a hard disk or diskette is to set up the space divisions on the medium and initiate a space allocation table that will know exactly how to reach each bit of data that may be stored there later.