the posts above do not seem to explain eddy currents
timmys tutoring version:
first of all we have changing magnetic fields. these changing magnetic fields induce some sort of changing electric field/current. (remember electromagnetic induction; like space/time intracacies, so is magnetism and electricity connected)
*cough anywayz. these changing electric currents induced, when in metals are called eddy currents. they are circular.
so now we move to lenz's law. at its basis, it dictates which way the eddy currents will flow. there should be another dot point on the relationship between len'z law and conservation of energy. but basically the others have said above, the polarity of the eddy currents will be such to oppose the change that caused em.
this is because if they did not oppose the change that caused em, the associated magnetic fields with the eddy currents would "reinforce" the original magnetic field, which would produce/induce even bigger eddy currents, leading to a infinite eddy current... which we know is impossible
hope that helps
*goes back to uni chemistry and shrodinger's equation, *rubs hands in evil glee