The definitions of the various caterories of approach go something like this (if my memory and a few old notes serve)
Category I - A precision approach and landing with a decision height not lower than 200ft and with either a visibility not less than 800m, or an RVR not less than 550m
Category II - A precision approach and landing with a decision height lower than 200ft but not lower than 100ft, and an RVR not less than 300m
Category IIIA - A precision instrument approach and landing with a decision height lower than 100ft and an RVR not less than 200m
Category IIIB - A precision instrument approach and landing with a decision height, if any, lower than 50ft and an RVR less than 200m
Category IIIC - A precision instrument approach and landing with no decision height and no RVR minimum specified.