"a digital file format used to store, transmit, and manipulate scientific and other images. FITS is the most commonly used digital file format in astronomy. Unlike many image formats, FITS is designed specifically for scientific data and hence includes many provisions for describing photometric and spatial calibration information, together with image origin metadata.
"A major feature of the FITS format is that image metadata is stored in a human readable ASCII header, so that an interested user can examine the headers to investigate a file of unknown provenance. Each FITS file consists of one or more headers containing ASCII card images (80 character fixed-length strings) that carry keyword/value pairs, interleaved between data blocks. The keyword/value pairs provide information such as size, origin, coordinates, binary data format, free-form comments, history of the data, and anything else the creator desires: while many keywords are reserved for FITS use, the standard allows arbitrary use of the rest of the name-space" (Wikipedia article on FITS, accessed 03-24-2010).
Because of its special features FITS became a very useful format for the long term preservation of digital images. It was also adopted by NASA as a standard, and was also adopted by the Vatican Library.