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A superconductor is a substance that conducts electricity without resistance when it becomes colder than a "critical temperature." At this temperature, electrons can move freely through the material. Superconductors are different from ordinary conductors, such as copper. Ordinary conductors lose their resistance (get more conductive) slowly as they get colder. In contrast, superconductors lose their resistance all at once. This is an example of a phase transition. High magnetic fields destroy superconductivity and restore the normal conducting state. Some examples of superconductors are the metals mercury and lead, ceramics and organic carbon nanotubes.
Normally, a magnet moving by a conductor produces currents in the conductor by electromagnetic induction. But a superconductor actually pushes out magnetic fields entirely by inducing surface currents. Instead of letting the magnetic field pass through, the superconductor acts like a magnet pointing the opposite way, which repels the real magnet. This is called the Meissner effect, and it can be demonstrated by levitating a superconductor over magnets or vice versa.