For a work to be copyrightable, the work must put into use an original idea.  An idea by itself is not copyrightable.  Rather, it is the physical use of that idea, such as an illustration or a written novel that is covered under copyright law.  Generally, a copyrightable work will fit into one of these eight categories: literary work, musical work, dramatic work, choreographic work, pictorial, graphic and sculptural (PGS) work, audiovisual work, sound recordings, and architectural work.