From what I understand, JK Rowling had some involvement in the creation of the Harry Potter movies so this post could relate to the books as well. The spells of the Harry Potter Movies are most often displayed as beams of energy (or some other shape made of energy):
(On left Voldemort casting the Killing Curse and on left Harry casting the Disarming Charm)
However if this is true and it is energy, or even plasma, then it would probably break the Law of Conservation of Energy, or if it's plasma then the Law of Conservation of Mass. I suppose it is a world with magic, so what do you think: is there a good explanation, or must we accept that the explanation is simply that it's magic?