At the heart of the fuse is the filament. The way the fuse is designed is set up so that, when it is hooked up to a circuit, it carries the entire amount of current passing through that circuit.
The metal filament within the fuse is designed to operate at a certain current level. If the current level is exceeded, the fuse will become too hot and, either the metal strip will melt outright or one of the solder joints attaching the metal strip to the conductors on the fuse will melt. Either way, the circuit will be broken, preventing a potentially dangerous overcurrent condition.
Fuses have different designs and are made out of different materials, depending upon the role they are intended to be used in. In some cases, they are provided with a spring that causes them to break the circuit more rapidly.
Fuses are oftentimes bundled together in a fuse box. This provides a centralized location for all of the circuits in a system. Fuse boxes also provide protection for operators from being brought into accidental contact with a live electrical circuit.