The fuse is one of the earliest electrical safety devices. It takes the form of a thin strip of metal with a low melting point, carefully designed so that it will heat up and break if greater than the rated amount of electrical current is passed through it. This is important because too much current, caused by a short circuit or a power surge, can damage electrical equipment and even cause fires.
In general concept, a fuse is very similar to a circuit breaker. They are both circuit protection devices that break the circuit, stopping electrical current from flowing, if they detect too much of the current. They both break the circuit faster the higher the current is. It can take several seconds when just at the rated current, or mere milliseconds at very high currents.
When designing power distribution systems, the circuit protection device closest to the appliance being powered should have the shortest break time, and the larger ones farther upstream, acting at higher currents and usually higher voltages, should have longer break times. This encourages the smaller circuit protection devices powering fewer devices to break if possible rather than the larger ones potentially protecting more devices. The longer the break time is, however, the more dangerous a potential arc flash event can be. Break times are seldom longer than a few seconds even at the highest levels, and fractions of a second at the lowest. Certain devices, such as motors, solenoids, and other things which experience a high inrush current when first switched on, can cause a nuisance trip if the fuse is too sensitive, however, and special slow-blow fuses are available to protect them.
Where a fuse physically breaks and must be replaced, however, a circuit breaker's spring-loaded switch is tripped (usually by heat induced by the current) and can be reset, either by hand or remotely with an electromagnetic solenoid. While this is more convenient, the fuse is a simpler design, more reliable, and faster-acting. Due to the convenience, circuit breakers are much more popular in 110 and 220 Volt commercial and residential circuits.
A common misconception is that fuses protect the user of the electrical equipment. In fact this is only indirectly true. A typical household fuse in a fuse box is rated at about 15-20 Amps, and this over 100 times the current necessary to kill a person. In fact, fuses are intended to protect the electrical equipment and wiring. While it will indirectly protect the user from exploding hair dryers, it won't help much if you drop the hair dryer in the bathtub. A ground fault circuit interruptor, which is much more sensitive to this sort of problem, is necessary to provide protection in such cases.
Fuses come in three common shapes. The most familiar shape for most people is a short, screw-in cylinder that looks a little bit like the base of a light bulb. The top is clear so you can peek inside and see if the fuse strip is broken. These are widely used in home fuse boxes. The second most familiar shape is a small, flat, U-shaped automobile fuse. Most of it is clear plastic, but the two ends of the U are bare metal to clip to the fuse box in the car. The clear plastic body makes it possible to see the fuse strip to see if it is broken. Most kinds are designed so they can be tested without pulling them out. The third kind is a longer cylinder with metal caps or blades at each end which clip into a fuse holder. These are widely used in power distribution, due to simplicity and reliability, and in small electronics, due to compact size at low currents. The body is often clear plastic or glass in these models so you can see if the fuse strip is broken.
While most people think of fuses as protecting their homes' wiring from up in the fuse box, there are certain cases in the US where fuses can be found in electric plugs. Surge protector power strips, for example, will usually have a fuse or circuit breaker in them (as well as the metal-oxide varistor that provides the actual surge protection). Furthermore, Christmas lights will usually have a small fuse in the plug end of the cord to protect the wiring and the small, delicate bulbs, which are too fragile for a hefty 20 Amp wall fuse. In the United Kingdom, all electrical plugs come with fuses in them.