Circuit breakers provide a means with which to turn on and off the power in an electrical system while providing over-current protection. What sets circuit breakers apart from switches and fuses is that they have the ability to be reset, controlled remotely, and have logic functions as part of there operation. Switches open and close and can even be equipped to be operated remotely but in themselves do not have the ability to provide overcurrent protection. Fuses provide overcurrent protection and the combination of a switch and a fuse can give open and close capabilities but the combination would still lack the ability to reset and continue to provide power and protection. These features make circuit breaker great candidates for anywhere in your system that may see frequent faults, the need for immediate power restoration is important, or where logical input are desire for different states of the power.
Circuit breakers range in complexity from a simple thermo-magnetic trip function that uses the heat produced by the current flowing through the breaker to instigate a trip to micro processor controlled protective devices with almost infinite adjustability and configurations. More about protective devices later, but suffice to say if you can imagine it modern microprocessor protective devices make it possible. Low voltage circuit breakers typically have their protective trip annunciation built into the frame or chassis of the breaker, designed and installed together from the factory. Medium and High Voltage circuit breakers are dependant on external or cabinet mounted protective relays to annunciate their trips. These devices can be specified as part of the breaker from the factory or part of a much larger substation protective scheme that includes differential, and distance feeder protection.