The good thing about breakers is that they can be thrown more than once, unlike fuses which must be replaced. One type of breaker uses a bi-metal strip that curves when heated. When the current becomes too high, the bi-metal strip bends, and moves the contacts apart, and cuts off the circuit. Once it cools off, it will move back, and the contacts will close again. Some have a wire wrapped aroun the bi-metal strip to heat it up, so the strip remains open after the breaker has been thrown. Once the current is cut off in the circuit, the wire around the strip will no longer hold the strip open, and the breaker will reset itself.
Another type of breaker uses an electromagnet to open a pair of contacts and shut off the circuit. When the current going through the breaker reaches a certain point, the electromagnet will produce a strong enough magnetic force, and pull the contacts that complete the circuit apart. A stop will then move into place, holding the two contacts apart. Once the stop is released, the breaker is reset.
NOTE: CIRCUIT BREAKERS SHOULD NOT BE USED IN PLACE OF SWITCHES! INCORRECTLY USING CIRCUIT BREAKERS AS SWITCHES CAUSES ARCING ON THE CONTACTS, AND CAN CAUSE DAMAGE TO THE CIRCUIT BREAKER.