From Science Europe Data Glossary
Definition: data format (or file format) is a standard way that information is encoded for storage in a computer file.
Explanation: the data format specifies how bits are used to encode information in a digital storage medium. Formats may be either proprietary or free and may be either unpublished or open. One popular method used by many operating systems, is to determine the format of a file based on the end of its name — the letters following the final period. This portion of the filename is known as the filename extension.