IRAF is the "Image Reduction and Analysis Facility". The main IRAF distribution includes a good selection of programs for general image processing and graphics, plus a large number of programs for the reduction and analysis of optical and IR astronomy data. Other external or layered packages are available for applications such as data acquisition or handling data from other observatories and wavelength regimes such as the Hubble Space Telescope (optical), EUVE (extreme ultra-violet), or ROSAT and AXAF (X-ray). These external packages are distributed separately from the main IRAF distribution but can be easily installed. The IRAF system also includes a complete programming environment for scientific applications, which includes a programmable Command Language scripting facility, the IMFORT Fortran/C programming interface, and the full SPP/VOS programming environment in which the portable IRAF system and all applications are written.

This is a cut and paste from the IRAF homepage, basically because they explain what it is better than i can. IRAF is freely available, but you proably don't want to use it unless you are doing astronomy in which case you proabaly already have it. IRAF has a command line interface which obeys some UNIX commands but not others, just to be paticullarly confusing. For more information have a look at their hompage