FITS is an image data file format for encoding astronomical data. The WCS (World Coordinate System) conventions map elements in data arrays to standard physical coordinates in the sky. FITS has provisions for image metadata encoded in an ASCII header at the beginning of files.
In July 2008, the IAU FITS Working Group offically approved the new 3.0 version of the FITS Standard document.
An extension of FITS that enables data to be defined to specify physical, or world coordinates within each pixel in an image. The conventions were orignally proposed in 2002 then incorporated into the 3.0 release of the FITS standard.
ESA is an international organisation with 20 Member States that coordinates the financial and intellectual resources of it's members to facilitate space related programmes and activities. Image data is released using FITS for many missions.
The High Energy Astrophysics Science Archive Research Center (HEASARC) is the primary archive for NASA's (and other space agencies') missions dealing with electromagnetic radiation from extremely energetic phenomena ranging from black holes to the Big Bang.
The International Virtual Observatory Alliance (IVOA) is an organisation that debates and agrees to technical standards that are needed to make the Virtual Observatory possible. FITS is one of those standards.