A circuit breaker is essentially a combination of a thermostat and a switch. The circuit breaker has a bimetal strip which heats and bends during a circuit overload. This bending action trips the breaker and opens the switch, thus breaking the circuit.
As soon as the metal strip cools (about two minutes or so) the breaker can be manually reset completing the circuit again. Most units reset either by flipping a switch or by pushing on the breaker. Since there is no easily removed part like a fuse, they are less likely to be replaced with the wrong size if frequent tripping occurs in a circuit.
Circuit breakers have gained popularity because of their convenience. Their one drawback is that the metallic contacts in some hostile environments can corrode shut so that the breaker does not open the circuit when it should.
Anytime a circuit breaker opens, it is an indication of a problem. The problem should be located and corrected before reclosing the breaker to ensure safety.