A Ground-Fault Circuit Interruptor
is a safety
device used in electrical outlets present in hazardous locations such as bathrooms, garages, and kitchen
A GFCI checks for situations when electricity flows directly from the hot wire into ground. A large percentage of electrocutions happen between the hot wire and ground, so interrupting the circuit when such a flow occurs is a good way to prevent electric shock.
In normal situations, the current flow in the hot and neutral wires of a recepticle is equal. All power "supplied" by the hot wire should "return" through the neutral line. If someone drops their hair drier into the bathtub, a good deal more current will flow through the hot wire than the neutral wire (because so much of it is leaking to ground through the water). A GFCI will sense this and trip, thus saving the idiot who decided drying their hair in the tub was a good idea.
GFCI's are now required by the National Electrical Code on all outlets in the vicinity of a source of water. They can usually be identified by the "test" and "reset" buttons on them.