International Terrestrial Reference System and Frame
The International Terrestrial Reference System describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be described as a set of procedures for creating a realization of that standard.
An International Terrestrial Reference Frame (ITRF) is a realization of the IERS. Due to experimental error, any given ITRF will differ very slightly from other realizations of the ITRF.
Practical navigation systems are in general referenced to a specific ITRF, or to their own coordinate systems which are then referenced to an ITRF.
The ITRS is maintained by the International Earth Rotation and Reference Systems Service (IERS).
-- need lots about the details of ITRS and ITRF