I would have to say that it due to the difference in the voltage that each needs supplied to the appliance and the amount of voltage ceoming from the outlet. I would figure that the 3-pronged outlet would produce the highest amount, and the smaller ones lesser. Then the two pronges ones with the difference in sizes would probably be a way of determining the current direction. That's just my hypothesis!

Its due to the difference in voltages that necessitates difference in the little holes. As for the larger holes, its for grounding the equipment. Some equipment may not need grounding, that its internal circuit is very simple. Or some may have inbuilt ground base. In these cases, the third outlet will not be present.

OK.... Three prong plug and receptacle: The third prong, the fat round one, is a ground prong. In equipment/tools with accessable metal cases or parts which can be handled by people as they operate, there is a ground wire attached to the metal case of the tool. This wire is attached to the big, fat, round, third prond. When you plug this device in, it connects the tool to ground. Litterally, the ground, or the return path for electricity. Electricity likes to take the path of least resistance. So, if you have a damaged or defective tool where one of the 'hot' wires inside the tool makes contact with the metal case of the tool, the electricity will pass through the case, down the ground wire, and to ground instead of passing through the body of the person holding the tool. This way a person is not likely to be electrocuted by a damaged or defective tool Now, for the part about one blade of a plug/receptacle being wider than the other (a 'polarized' plug/receptacle): Back in the good old days, before the three prong receptacle became standard, outlets were constructed with one side built to take a wider blade than the other, and plugs were made to suit this arrangement. This is so that the plug would only fit in the receptacle one way. When these plugs are wired up, the 'hot' wire would be connected to the narrow side on the receptacles and the ground return connected to the wide side. In tools and equipment where you may be exposed to shock hazards vie a metal case, the metal case would have the wide blade side of the plug connected to the case. Again, if the 'hot' wire in the tool touched the case, the electricity would take the path of least resistance through the power cord to the wide bladed side and to ground, instead of the person holding the tool. Some devices are constructed such that there are no exposed metal parts for people to touch. In these cases, they do hot have to have a polarized plug on them, as it makes no difference which way they are plugged in for safety. Obviously, these plugs and outlets must be wired correctly in order to perform these functions safely. It is also very important that they not be modified either (like cutting off the ground plug on a power cord so it will fit into a 2-prong outlet), because this reduces their safety.

Non-Polarized outlets have two vertical slots side by side that are the same size. Which one is hot and which one is neutral? Keep your family entertained for hours guessing! Loser gets electrocuted sticking a knife into a toaster. These are no longer used, and modern polarized plugs will not fit in them. That is, unless you file down the wider prong so it will fit, in which case you deserve to be electrocuted. Polarized outlets are different in that the slot for the neutral wire is wider than the slot for the hot wire. This makes it difficult to insert the electrical plug the wrong way, although I wouldn't put it past some people. The purpose for this is most easily seen in devices such as toasters and lamps, which have exposed parts that can have electrical current running through them. A lamp, for example, powers the bulb both through a button on the bottom and the body of the screw the light bulb fits into. Since the screw fitting is large and easy to accidentally touch, in a properly wired polarized outlet, the screw fitting will be connected to the neutral wire. This is much safer because the neutral conductor, also called the grounded conductor, should always be at zero volts with respect to the idiot changing the light bulb without turning off the power first. This makes it much less likely to deliver an electrical shock. Polarized, non-grounded outlets are becoming increasingly rare these days. source: http://everything2.com/node/52057

the third prong is the ground

In North America, the 3 prongs on a polarized, grounded outlet are:          Hot (smaller): Voltage constantly changing from -170 to +170   Neutral (longest): Stays at 0 Volts  (& connects to a safe ground) Ground (smallest): Directs current away in case of emergency. 60Hz AC is alternating current: this means the hot wire is positive for 1/30 second, but then negative for the next 1/30 second & so on (while neutral stays at 0 Volts). This is why it does NOT matter which way you insert a non-polarized plug, the polarity is continually changing every 1/30 second anyway! While it doesn't matter to many appliances, it does matters to humans. You do NOT want to come in contact with the hot while you are grounded. This is why circuits are designed so that the hot wire only connects it's voltages to surfaces that can't be touched once powered on. In light bulbs: the touchable shell connects to neutral & only the tip connects to hot. Laws govern which appliances, etc must be either polarized or grounded. I've oversimplified a little (w/o mentioning center-tapped transformers, etc) but if you are curious: http://en.wikipedia.org/wiki/Domestic_AC_power_plugs_and_sockets#The_three_contacts http://en.wikipedia.org/wiki/Split-phase_electric_power http://amasci.com/amateur/whygnd.html