An idealized coordinate system that is independent of epoch, ecliptic, or equator. For ease of use it has been aligned close to the mean equator and dynamical equinox of J2000.0. The origin of the coordinates is at the centre of mass of the Solar System, i.e. they are barycentric. A reference system differs from a reference frame in that it lays down the specifications for a coordinate system, whereas a reference frame puts those specifications into practice; the International Celestial Reference Frame is the practical realization of the ICRS in the form of a list of coordinates of specific celestial sources.

http://aa.usno.navy.mil/faq/docs/ICRS_doc.php Information page at the US Naval Observatory.