Really, 3 prong plugs are generally installed on appliances that draw a large amount of current, or are prone to exposing HUMANS to electrical shock. The third prong is a ground prong that connects to the bare copper wire you find in a sheathed electrical cable cable. That wire is in turn connected to a "grounding rod" which is usually a 6' or longer metal rod driven into the ground, hopefully deep enough to find water. The concept here is that if electricity needs to find a place to go, you would rather it goes into the ground than into you. The grounding plug provides the most "attractive" route possible for the electricity, thus saving your heart from a very unpleasant experience.
So, you ask, why doesn't everything have a three prong plug? In product design, appliances that perform to certain specifications (again, I am not an electrician, nor do I know the specifics here) qualify for 2 prong plugs. Typically, you will see things like electric drills that advertise that they are "double insulated" which means that there are at least 2 major jumps that the electricity in the motor and switch assembly would have to make before it could find its way out to the outside of the tool casing. This is not a free ticket to operate the tool while standing in a puddle, nor is it an excuse to not use a GFCI circuit in your garage or kitchen.
The most important thing to remember is that if a plug has 3 prongs, it is because the appliance NEEDS that third prong so it can PROTECT YOU. If you are faced with a 2 prong outlet, DO NOT CLIP THE GROUND PLUG! Just use an adapter and BE CAREFUL.
(All the information here is presented by someone who is NOT a licensed electrician. Nothing replaces common sense and due caution.)