Electromagnetic induction is the principle (revolutionary when discovered in 1831), that states that a voltage can be induced in a wire or other conductor that is moving in an external magnetic field; if the conductor is part of a circuit, a current will flow. This phenomenon likewise occurs when it’s the magnetic field that is moving or changing, and the conductor that is stationary. This would be the case, for example, if you move a magnet over a coil of wire, or if the field is increasing or decreasing, as in the case of an electromagnet fed by an alternating current.
The following tutorial illustrates the first scenario, in which the conductor is moving.
A Pendulum has a Copper Coil on one end and a small Lamp at the other; the two ends are connected via wires running through the arm of the pendulum. At the bottom of this apparatus is a Horseshoe Magnet. The lamp is not lit when the pendulum is at rest, the copper coil between the north (N) and south (S) poles of the magnet (the field of the magnet is flowing from the north to south pole). Click the Swing button. The lamp flashes briefly as the coil passes through poles of the horseshoe magnet. The motion of the copper coil through the magnet induces a current in the coil, causing the lamp to light. The lamp goes out when the copper coil is not passing through the magnet. Notice also that the lamp grows dimmer as the pendulum slows. The slower the pendulum, the less dramatically the magnetic field changes, resulting in less and less current generated.