A copyright is a set of legal rights that an author has over his or her work for a limited period of time. Copyright insures that the person who created the product is reimbursed for his intellectual work. Books, music, images, sound files, and many other things are copyrighted.
Granted, it is easy to download songs from the Web or make copies of a book instead of buying it. But no matter how easy it is, those works are copyrighted to give economic incentive to the creators of these works. In other words, if we don't buy the item the creator goes unpaid for his work.
OC's acceptable use policy details the expectations for OC students and copyright.