More advanced circuit breakers use electronic components (semiconductor devices) to monitor current levels rather than simple electrical devices. These elements are a lot more precise, and they shut down the circuit more quickly, but they are also a lot more expensive. For this reason, most houses still use conventional electric circuit breakers.
One of the newer circuit breaker devices is the ground fault circuit interrupter, or GFCI. These sophisticated breakers are designed to protect people from electrical shock, rather than prevent damage to a building's wiring. The GFCI constantly monitors the current in a circuit's neutral wire and hot wire. When everything is working correctly, the current in both wires should be exactly the same. As soon as the hot wire connects directly to ground (if somebody accidentally touches the hot wire, for example), the current level surges in the hot wire, but not in the neutral wire. The GFCI breaks the circuit as soon as this happens, preventing electrocution. Since it doesn't have to wait for current to climb to unsafe levels, the GFCI reacts much more quickly than a conventional breaker.
All the wiring in a house runs through a central circuit breaker panel (or fuse box panel), usually in the basement or a closet. A typical central panel includes about a dozen circuit breaker switches leading to various circuits in the house. One circuit might include all of the outlets in the living room, and another might include all of the downstairs lighting. Larger appliances, such as a central air conditioning system or a refrigerator, are typically on their own circuit.
For more information about circuit breakers and other electrical systems, check out the links below.