Eddy currents are electrical phenomena that take place when a conductor is exposed to an oscilation of the magnetic field due to the relative motion of the field source and conductor; rewind. You have a conductor, say a copper tube, and a magnet. One moves relative to the other and you've got current (basically a circulating flow of electrons). These currents also generate heat as well as electromagnetism, and can be harnessed. I won't go into additional details which you can find on wikipedia or physling, but instead, I'm gonna show you this video, which is just another example of how amazing physics really is.
