Where do I need it?

Any outlet within six feet of a sink or tub/shower must be protected by a Ground Fault Circuit Interrupter. That means kitchen outlets, and bathroom outlets - including the one built into the medicine chest, and the outlets around the wet bar. In addition, outdoor outlets and those in the garage which are not "dedicated" must also be protected. You can also put GFI outlets in other places where they will make you and your family safer. The kids´ bedroom and the game room jump to mind. Also, see the categories Outdoor Lights, and Safety.