on a standard 3 prong plug the left (larger) prong is "Neutral" the right prong is Hot (this is where the electricity comes from) and the ground prong is well a ground. If you trace the wires back to your breaker box you will find that the neutral and ground prongs are connected to the same thing, your houses ground. So whats the difference? the ground plug is usually connected to the casing of an electronic device that has the potential to shock the user if something were to come loose inside.

With all that said as far as herpstats go the ground isn't necessary, but it is only $10 more if I remember correctly so thats what I got just to add an extra layer of safety.