Florida State University defines Lenz's Law as an induced electromotive force that generates a current that induces a counter magnetic field opposing the magnetic field generating the current. Lenz's Law was first discovered by the Russian physicist Heinrich Lenz in 1834.
According to How Stuff Works, Lenz's Law applies to electric generators. A generator induces an electric current, and the direction of the current is such as to oppose the rotation of the generator. The more electrical energy a generator delivers, the more mechanical energy is required to turn it.
According to the Nondestructive Testing Resource Center, French scientist Dominique Arago developed an experiment to demonstrate Lenz's Law. The experiment involves hanging a magnet from a string over a conductive, nonmagnetic material like copper. The magnet swings back and forth over the surface but doesn't touch it. When the magnet swings over the conductive surface, its magnetic field cuts through the conductor and induces an electrical current, which generates its own magnetic field opposing the magnet's magnetic field. This causes the swing of the magnet to slow down even though the surface is nonmagnetic. If the conductive surface is taken away, the magnet swings freely because there is no current created.