GB, or Gingerbread is a all around OS for devices of all kinds (phones, eReaders, tablets etc.).
HC, or Honeycomb was made specifically for tablet devices due to their larger touchscreens.
ICS, or Ice Cream Sandwich, is the newest official release of the Android OS, and is basically slated for all newer devices again, like phones, tablets and others. This is assuming that the hardware in the device is capable of running the new OS properly without any hiccups.