Copyright is the property of the creator of their works. It protects the author so they are able to claim it as their own. It is illegal to copyright someone else's work and claim it as your own. When using someone else's work, the student or teacher must cite their sources and give credit to the owner.
Fair Use is using someone else's work to "transform" it into something else. It provides a reasonable use to use someone else's work such as commentary, criticism, or parody.