Copyright is a term that is used when someone has full ownership of their work. It is illegal to copy another persons' work and try to use it as your own. This is also called cheating and is against the Acceptable Use Policy.
Fair use gives us the opportunity to use the work that someone else has created because they have given us those rights. It is necessary to make sure we still credit the work that the person has done. Fair use would be like using a free online worksheet.