One of the many perplexing things about modern household appliances is why some of them come with three-pin plugs and others with only two. The difference between these two plugs is significant. Understanding them is just a matter of knowing what to look for.
The reason that some plugs come with three pins is that they are "grounded." This means that the third pin connects directly through a series of wires to the ground outside of the building. In case of an electrical problem, you don't want the electricity seeking to ground inside your home, as this could cause a fire; it is safer to have it grounded outside.
Appliances without three pins generally don't need enough electricity to require a ground. Even in the worst case scenario, a problem with these electronics would only provide you with a mild shock.
Cutting the Third Pin
Most electricians recommend leaving the third pin attached to the wire on an electrical plug. It is there for your safety. Although it can be a hassle to work around, it is much safer to leave it in place.