What: Copyright is the complete ownership of something and the owner must be attached to it at all times. Fair Use is the ability to use said copyright in the aspect of education or for approved reasoning.
Why: Someone could use a copyright when they need to cite an exact example OR if they own the material. Someone may also have Fair Use to other things because they are not out right owned.
Consequences: If found to have gone against copyright laws you may be subject to legal consequences in the form of fines.