The fuse breaks the circuit if a fault in an appliance causes too much current to flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit. The main difference between circuit breakers and fuses is that the latter are single-use and need to be replaced afterwards.

Source: BBC Bitesize