Since I'm an Electrician...
pbroy's Lesson on Electronics for the Day:
Prong count doesn't matter when it comes to how much a cable can power a device. The wire gauge thickness is what matters. The thicker the wire, the more Amps it can carry.
This is your typical outlet set up in an American home:
The long slot on the left is Neutral. The slot on the right is Hot. The hole on the bottom is Ground. You might be asking yourself, "shouldn't everything be grounded?" Well, it is. The Neutral is grounded at the breaker box. If you stick a metal object into that long slot on the left, you shouldn't get shocked. I'm not telling anyone to do this, and please don't, cuz you wouldn't know if the Electrican who wired the house did it properly. You will need a test tool to figure that out.
So what is the purpose of that ground pole then? It is typically used for Electronics that have a metal housing. The ground wire is connected straight to that chasis. It's a fail-safe for if a hot wire comes loose and touches the chasis. If the chasis is hot and someone touches it, they will get shocked or killed. You will see this on things such as toasters, TVs and PCs.
The pictures below do not represent a 3 prong setup for power:
