A copyright is a legal certification that a certain person or company owns a creative work. No one is allowed to distribute or reproduce the work without the copyright owner’s permission. If you create a written text or one of the other kinds of works eligible for copyright, you are its owner even if you do not formally register a copyright for it with the U.S. Copyright Office. Why, then, do people spend the effort in registering copyrights if you still own your work without them? In some situations, registering a copyright prevents others from taking credit for your work and profiting from it.
via Taking Ownership of Your Original Work: Reasons for Copyright Protection — A People’s Choice