A fuse protects against large currents flowing by breaking the connection.
Fuses can be a good idea
- ...when such a current is not unthinkable
- ...when this is good for safety, in that a fuse burning is a lot more controlled than many other components burning out
- ...when the fuse costs less than the components cost they protect (and/or the maintenance cost) - which is probably a lot, more often than fuses are used in practice.
Basic (melting) fuses
A very simple fuse in household items is a thin resistive wire which will simply heat up and burn through at some current.
Fuses come in various speeds
- common fuses may take perhaps a second
- fast-blow fuses may take perhaps a tenth of a second.
- Useful when regular fuses are too slow to actually protect something delicate/expensive.
- Still don't protect quite everything, though
- slow blow fuses / time-delay fuse may take seconds or tens of seconds to blow. They are used when short spikes are to be expected and acceptable.
Note that when replacing a fuse, even with the correct rating, if the speed characteristic is different you may still get fuses burning when everything is okay (if you replaced a slow with a fast) or not protecting components as well (if you replaced fast with slow).
Fuses often come encased in something (glass, plastic, ceramic). This is mainly for safety, to not have the wire vaporize into air.
Glass fuses shouldn't be used with high voltage or high current, because they can vaporize the fuse and continue conducting some current, and/or arc after blowing. In these cases ceramic fuses are much safer - they have ceramic walls, and sand filling.
Thermal fuses are a variation that break in reaction to heat external to them (rather than from their own resistance), seen for example in hairdryers.
Resettable fuses / polyfuses / semifuses / PTC fuses
...and also known under at least half a dozen brand names.
Any design that cuts out the circuit at some amount of current, but which will conducts again once power is removed (and restored to the acceptable level).
Often a specific design of thermistor (non-linear, polymeric PTC (PPTC)). When it sees more current than it is rated for, heat causes it to become a high-resistance element (on the order of a few kilo-ohms), which in most applications is equivalent to cutting off the current.
Polyfuses do not trigger very fast, so current will be allowed for a short time.
Arguably polyfuses are better at things like protecting elements against heat damage from long-term overcurrent, and protecting battery chemistry against short circuits, than they are at protecting sensitive ICs.
They are not necessarily a complete protection in themselves, in particularly not so good against damage from short voltage spikes, but they can be good against accidental large draws and short circuits.
Note that PPTCs have higher resistance of themselves than simple burn-through fuses.
Used where replacement is hard or annoying, say, space stations and USB ports. USB specifies that ports should be able to take short circuits without having the USB port be broken forevermore - which is nice because motherboard USB ports would be hard to replace, particularly in laptops. (In practice, resetting these fuses may be bothersome, sometimes requiring complete removal of power -- wallpower, laptop battery, and waiting a few minutes for possible capacitors to discharge)
Originally referred to large (often knife-style) switches.
After that, the term referred to large burn-through fuses, often in ceramic containers, that had to be replaced.
Recently they are usually switches that trip themselves off when they sense more than a certain current (safety-ground setups), and have to be manually switched back on.
The most common design uses a solenoid to pull on a trip mechanism. Other designs exist, in part because different voltage/current needs are better served by different designs.