A superconductor is a material that, when cooled below a certain temperature, has no electrical resistance.

What this means is that superconductors may carry an electric current without the need for any voltage to drive it. For example, a ring of superconductor may carry an electric current essentially forever without the current decaying in any way. In a similar ring of the highest quality copper, the current would decay away in a small fraction of a second. In fact, the vanishing electrical resistance of a superconductor is only one of many properties unique to superconductors.

Another and more fundamental effect is the Meissner Effect, in which the superconductor expels the magnetic field from its interior. However, the zero resistance property is clearly of fundamental technical importance so it is at first puzzling why superconductors are not used more widely.

The reason they are still confined to niche applications is not that they are particularly rare (common materials like aluminium, tin, lead etc are all superconductors), it is that they require low temperatures in order for the superconducting state to appear.

The reason they are still confined to niche applications is not that they are particularly rare (common materials like aluminium, tin, lead etc are all superconductors), it is that they require low temperatures in order for the superconducting state to appear.