Washington State University: What copyright does not protect
This page does a wonderful job of explaining what cannot be protected, and thus gives a full complement to the principles of copyright.
Briefly:
* Materials Not Creative Enough
* Idea/Expression Dichotomy
* Expired Copyrights ("Public Domain")
* Works Copied as Fair Use
Each of these headings is given a concise explanation.
http://publishing.wsu.edu/copyright/what%5Fcopyright%5Fdoes%5Fnot%5Fprotect/