Residual-current device
A standard circuit breaker protects the house electrical wires, receptacles and switches, and appliances and fixtures from dangerous conditions, which could contribute to overheating and possible fire.  A GFCI / RCD on the other hand, protects people from shock and electrocution and is often found in bathrooms or kitchens, but more specifically where electrical devices may be used near sources of water. While using electrical devices in these locations, someone's bare skin may be in contact with water or a wet floor or metal fixtures.  Any of these can provide an alternate path for current to travel in the case of an electrical fault.