A pyrotechnic fuse, in military and engineering usage, is a timing device, working on the principle of controlled-rate burning in a linear medium, which is most often associated with the delayed initiation of a detonator. Fuses are most commonly of cloth impregnated with chemicals to sustain burning and control the propagation rate of the flame.
Prior to the 20th century, gunners of manual cannon would hold a length of burning "slow match" or "slow fuse" (not at the burning end), and, when it came time to fire the piece, would apply the burning part either to gunpowder or "fast fuse" in the touchhole of the gun, which would then carry flame to the main propellant charge.
In a modern hand grenade, when the safety ring is pulled, it releases a spring-loaded mechanical lever, often called a "spoon", in the fuze assembly. When the lever free to move, it strikes a friction-sensitive surface that causes an internal burning fuse to light and then trigger the actual explosive detonator.
"Instantaneous fuse", perhaps better known by the trade name Primacord, is still pyrotechnic, but the cloth or plastic supporting structure conveys an explosive rather than flame wave.
Fuses may be used not to trigger an explosive, but, when burned through, to release a mechanical or electrical actuator of some sort. The line is blurry, as such things as pyrotechnic fasteners use a fuse to trigger a tiny explosion, enough to sever a bolt but often staying inside a housing.