Simply put, a breaker interrupts power in a circuit to protect it from a short circuit.
More technically, a breaker is an automated, rated (aka comes in sizes), mechanical device, that is designed to interrupt a circuit (cut it off), whenever the breaker senses current (amperage) at higher levels than it's rating. Okay the first sentence is easy enough to understand. But why would I ever want to use one? What are they for? WHY???
Breakers, like fuses, sense power consumption inside a circuit, and are designed to be the most fragile piece in the circuit. Whenever there is trouble, the breaker or fuse senses the problem and blows. So what is the difference between a fuse and a breaker?
Well, a breaker is more applicable than a fuse for many applications because it can be reset, either manually or autmatically, once the high current load has been removed. For instance, you put a few too many appliances on at once in your RV. This creates a high current situation for the system. To protect itself, the system wants to shut down. If you are camping and you blow a fuse, you have no more toys until you replace the fuse with a new one. If you blow a circuit breaker, you can wait until the system cools down or some of the appliances are remove, then reset the circuit breaker, and go back to having fun.
So then why would anyone ever use a FUSE? Shouldn't I always use a circuit breaker?
Well a circuit breaker is designed to protect things from overheating. Since overheating by way of electricity is not the same as blowing electronics with a spike of power, we really need two different devices. A circuit breaker is designed to protect wiring (like that in your home), motors (like the one in your disposal), battery chargers, inverters, and other power generation and switching equipment. This power equipment is not sensitive to spikes in power. Instead you are trying to keep these items from getting too hot and melting. A circuit breaker does this job nicely, but isn't sensitive enough to protect those fine electronics.