Some of the materials when their temperature falls below a certain value, can move in a state with zero electrical resistance. This temperature is called critical. Today there are a large number of compounds, elements, alloys, and ceramics having superconducting properties.
Superconductivity - the quantum phenomenon. Superconductors have the ability to push magnetic fields from its scope, this behavior is called the Meissner effect. The existence of this effect suggests that superconductivity can not be described as the perfect conductivity in classical physics.
The practical use of superconductivity at the boiling point of liquid nitrogen, cheap enough cryogenic liquid, began in 1986-1993gg., After the discovery of high-temperature superconductivity.