Copyright is a legal protection insuring that the creators or owners of a creative product (writing, drawing, music, film, etc) retain the rights to using and profiting from their work. Infringing on copyright can lead to court cases, fines and possibly jail time.
Creative Commons is an organization that grants free licenses clarifying the creator’s ownship while allowing others to use the work. A Creative Commons license does not replace copyright, but rather lets the creator pre-determine how it can be shared and used. In most cases, a work that holds a Creative Commons license can be used without fear of infringing copyright as long as credit is given to the original source.
Fair Use (sometimes called Educational Use)
The fair use doctrine allows limited use in specific situations of copyrighted material. The standards for what constitutes fair use depend heavily on context, but often boil down to the question of whether the copyright holder was deprived of their rightful income for the use of their material.
Plagiarism is rarely if ever a legal violation, but it is ethical misconduct. Plagiarism is presenting ideas, words, phrases or even images as your own instead of crediting their creation to the person who originated the material. In many cases, plagiarism is solved by correctly attributing the source.
When the copyright expires on a piece of work, the work is in public domain. Once something is in public domain, anyone may use it in any way they wish without threat of legal reprisal. The laws determining when items enter public domain change, and Mickey Mouse may be one reason why (see attached article). Even works that are in public domain, like the Bible, can have specific versions, notes or guides that are copyrighted, so it’s generally safer to assume a work is copyrighted unless you can verify it is not.
Resources for further research:
Art Law Journal article “How Mickey Mouse Keeps Changing Copyright Law“