An RCD is a device that immediately switches off the electricity when it detects leakage or flow of electricity to earth (or another fault path such as through a human body, for instance). This happens within a few milliseconds, long before electrocution can cause death. RCDs also prevent fires caused by faulty appliances by detecting electricity flowing to earth in electrical wiring and accessories.
No; a circuit breaker protects the wiring by disconnecting supply when a higher level of current flow is detected than can safely be carried by the cable. This is often not fast enough to prevent electrocution. A circuit breaker and safety switch look somewhat similar, but the RCD is easily identifiable by its test button.
Once every three months, use the test button on the RCD to see if it is working. This should immediately turn off the appliances connected to the power points (and possibly lighting) that the RCD runs through.