The Circuit Breaker is found in an electrical service panel and is an electrical device used to protect the electrical wiring from an overloaded (overcurrent) condition when exposed to more electrical current than it is designed to handle.
Similar in function to an electrical fuse which will blow when overloaded, the circuit breaker will turn off or "trip" when in an overcurrent condition. However, unlike the fuse which is rendered useless when it blows, the circuit breaker is not damaged when it trips, and can be reset.
For a more detailed description of circuit breakers and
how they work see The Circuit Breaker.