The NEC (National Electrical Code) requires that all receptacles controlled by a GFCI be identified. That is why you get many stickers saying GFCI when you buy a ground fault circuit interrupter.
The code requires that ground fault circuit interrupters be installed in all areas where a human might come in contact with the earth, directly or indirectly. That includes, but not limited to, the kitchen, bathrooms, all outdoor receptacles, and the garage.
The GFCI works by monitoring the current in both wires, usually black and white, and if there is a difference of more than 6 mA, it assumes the difference is going through a human and shuts the circuit off. It only takes about 15 to 20 mA to electrocute you. Anytime you are standing directly on the earth. You are standing on one side of a 120 V circuit. Hope all that makes sense.
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