A fuse is a strip of metal or a wire constructed of copper, zinc, silver or aluminum. Some fuses are made of alloys of these metals. Some fuses use two strips with one designed to melt quickly to prevent a short circuit and another designed to fail in the event of a long-term overload.
Fuses are designed to carry a set amount of current and melt rapidly when any excessive current is applied. Fuse wires must be as short as possible to avoid causing unnecessary resistance to the electrical system they are a part of while being long enough that the excessive current that breaks them cannot spark between the melted ends and destroy the system the fuse is protecting.