The About.com Home section explains that a circuit breaker trips, or fuses blow, because of either an electrical overload on the circuit or a wiring problem. Overloads are more common and easier to fix than wiring problems, which can be more complicated.
According to About.com, the most common cause for a circuit breaker to trip is an electrical overload; it causes the breaker to close the circuit and shut off the electricity in order to prevent the circuit from overheating. In this case, more amps are being used on the circuit than it is designed to hold. Moving appliances, such as lamps, radios or small electrical items, to another circuit in the house often solves this problem. When a circuit trips, everything on that circuit loses power. Turn off recently added electrical devices to reduce the load.
About.com explains that the other reason a circuit breaker can trip is from a wiring issue; usually a short circuit or a ground fault. In either case, turn off the circuit before attempting to find the problem. In a short circuit, a hot black wire often makes contact with another hot wire, or a neutral wire which is usually white. In a ground fault, the hot wire typically touches a metal piece in the fuse box. The power must be shut off before one removes the hot wire from its incorrect position, and then turns the circuit back on. Dealing with fuses and circuits requires sound electrical-safety practices and procedures.