A superconductor or in other terms a supraconductor, is a material that has a zero electrical resistance. Because of this, a current through it can flow an infinite time without being transformed by the material electrical resistance into heat. The state of being superconductor exists below a certain temperature, named critical temperature and below a certain value of external applied magnetic field (named critical field).
Superconductivity has been first time discovered at the beginning at the last century in Mercury (Hg) at liquid Helium temperature, when around 1910 Helium gas was liquefied (T= 4.2 K). Beginning 1950 scientists begun to study intensively superconductivity and new materials that become superconductor at higher temperatures (up to 90 K) were discovered. The theory that explains this phenomenon was named BCS (after the scientists names who explained it: Bardeen, Cooper, Schrieffer and who got a Nobel prize for it). In a few words explained, at low temperatures, single electrons that usually conduct electrical current couple two by two (each having opposite spin +/-1/2) forming a so called Cooper pair. Because now the total spin of the pair is integer, it acts as a boson, not as a fermion. (Bosons are particles having integer spin, fermions are particles which have half integer spin). Unlike Fermions that occupy each a single energy level (see Pauli exclusion principle), bosons can condensate all on a single energy level. Thus all the Cooper pair will be on the same level of energy on the bottom of the valence band having above a small energy gap (forbidden band) that separates them from the levels that electrons usually occupy. This energy gap, further prevents the Cooper pairs to be scattered by crystal nuclei oscillations (in scientific terms said "by lattice phonons") and thus the electrical resistance becomes zero.
Superconductivity is a quantum mechanical phenomenon. It is first observed in mercury by Dutch physicist Heike Hamerlingh Onnes. A Supercponductor is a material which does not resist the flow of electrons through it. They have the ability to conduct electricity without the loss of energy. When a superconductor is cooled below its critical temperature, its resistance drops to zero abruptly. This phenomenon can be explained with Meissener effect.
The Meissner effect does not cause the field to be completely ejected but instead the field penetrates the superconductor but only to a very small distance, characterized by a parameter λ, called the London Penetration Depth.
Magnetic-levitation is an application where superconductors perform extremely well. Transport vehicles such as trains can be made to "float" on strong superconducting magnets, virtually eliminating friction between the train and its tracks
An area where superconductors can perform a life-saving function is in the field of biomagnetism. Doctors need a non-invasive means of determining what's going on inside the human body. By impinging a strong superconductor-derived magnetic field into the body, hydrogen atoms that exist in the body's water and fat molecules are forced to accept energy from the magnetic field. They then release this energy at a frequency that can be detected and displayed graphically by a computer.