It is described as a medium sized energy production unit, a self-contained fusion reactor. It's the primary ammunition of all two-handed Energy Weapons in both Fallout and Fallout 2. It can also be used as fuel for the Highwayman. One cell gives a 50% charge (or 1% per remaining charge, in the case of partially spent cell.) It is fundamentally the same size as a 6 volt battery.
Can be bought at Fortress.