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. (though people also use it to refer to other things)
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 later, e.g. once power is removed and/or restored to the an acceptable level.
Used where fuse 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 wallpower or laptop battery, and ensuring PSU capacitors are empty
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 (in series with the real load) is equivalent to cutting off the current.
Polyfuses do not trigger very fast, so high current will be allowed for a short time.
Arguably, polyfuses are better at things like protecting components against heat damage from long-term overcurrent, and protecting battery chemistry against short circuits, and not so much at protecting sensitive ICs.
It may also not be good enough 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. This can matter to power efficiency.
Tangentially related: MOVs
A long time ago, circuit breakers referred to large switches you would manually switch, such as knife-style switches.
These days, circuit breakers often refer to switches that, on top op being manually operated, are also overcurrent devices: they trip when they sense too much current, protecting your house's wiring from overheating.
(The most common design uses a solenoid to pull on a trip mechanism. Other designs exist, in part because different needs are better served by different designs)