A residual current device provides much the same functionality as the ground fault circuit interruptor, that is, to detect stray currents from the hot wire to ground and break the circuit in such an event. An RCD works just like a GFCI -- it contains a small transformer which monitors the current on the hot and neutral wires it protects. If the current is not equal on both wires, this means that some current is being lost to ground, possibly passing through a human being to do so. When such a situation is detected, it opens the electrical contacts, shutting off the power. Despite the similarities, intended installation is somewhat different.
In the United States, where GFCIs dominate the market, a ground fault current interrupter is generally found built in to a circuit breaker or built in to an outlet. There are also models built in to extension cords designed to protect power tools when no GFCI protected outlets are available. These devices are generally rated for 20 Amps and protect only one device or one circuit.
In the United Kingdom, where RCDs are the norm, a residual current device is found in the consumer unit (analogous to a U.S. circuit breaker panel or fusebox) located on the feed side of the circuit breakers. It is a separate unit that provides no function other than to monitor ground faults and break the circuit. In some installations, they protect every circuit breaker, and in split-load installations they protect only some of the circuit breakers (the refrigerator and lighting circuits being excluded from RCD protection to prevent problems caused by nuisance trips). These units are typically rated 80 Amps or higher since they are intended to protect much more equipment.
The biggest advantage to the RCD is the simplicity and cost effectiveness of having only one device protecting the entire installation. The drawback, however, is that a ground fault on any one circuit will shut off the power to all the protected circuits, not just the one with the problem.