FITS: Flexible Image Transport System
A file format defined for astronomy data that is designed to be self-describing. A FITS file contains an ASCII header that includes metadata which describes the data, followed by one or more extensions containing the data.
The formal FITS definition leaves open many details that are needed for specific applications. The high energy astrophysics community has defined a number of formats within the FITS paradigm, e.g. RMF and ARF files. A convention for storing regions in FITS headers and a new way to store functions in the FITS header with a FEF (FITS Embedded Function) format have been created for CIAO.
See the GSFC FITS Support Office for more information on FITS in general, and for a brief introduction to FITS.