Ground Fault Circuit Interrupter(GFCI). Sometimes known as a GFI, or a residual-current device (RCD or RCCB). Can refer to receptacles as well as circuit breakers. They detect a leak in current, or an imbalance between the current on the hot and neutral wires. This is used to prevent electrical shock. Use with the [electrical] tag
Ground Fault Circuit Interrupter(GFCI) devices are usually required where electricity can contact water or some other ground, including kitchens, bathrooms, and outdoors. They work by detecting an imbalance (usually around 5mA) between the draw on the hot and the returning current in the neutral, and thus will disconnect(interrupt) the circuit. This prevents electrical shock. GFCI receptacles can protect down-circuit receptacles, provided those outlets are attached to the LOAD side of the GFCI.
The breakers may be installed within the breaker panel or as a receptacle or stand-alone, "dead-front" device in North America; they are typically found in the breaker panel in other parts of the world. In Europe, these may be called a Residual Current Device(RCD, usually for receptacles) or a Residual Current Circuit Breaker(RCCB), but they work the same way.
Similar to (but not the same as) Arc Fault Circuit Interrupters(afci), which are used to protect against arcing that could cause a fire. Some circuit breakers will do both GFCI and AFCI.