There is information about GFCI switches on OSHA.gov, HowStuffWorks.com, Levinton.com, and HomeDepot.com. GFCIs are ground-fault circuit interruptors, which protect people and equipment against electrical incidents.
A GFCI is a fast-acting circuit breaker that shuts off electric power within as little as 1/40 of a second in the event of a ground fault, which is a break in the low-resistance grounding path from an electrical system. It can protect against destruction of wire insulation, fires, overheating and electrical shocks. A GFCI monitors the amount of current flowing and trips the circuit if there is an imbalance. It offers protection in home, office and construction sites and wherever electrical equipment is used in a damp or wet location.