What does hot and cold mean on an AC outlet?

So I sometimes hear that the outlet has a smaller hole for hot and larger for cold so its harder to stick your finger in hot. But if it is an AC wave, shouldn't they both be the same, because they get 50% of pos. and neg? What exactly does Hot and Cold mean on an AC outlet and does it have something to do with ground? (ex: you touch hot and it goes through your body to ground and gives you a shock)