The U.S.Copyright Offices states that fair use is, "a legal doctrine that promotes freedom of expression by permitting the unlicensed use of copyright-protected works in certain circumstances." To learn more about fair use and the four factors used to measure it, check out Stanford University's Center for Internet and Society Fair Use Project.
The Public Domain refers to works that are not protected by copyrights or patents, and can be used by the public without obtaining permission.
Creative Commons offers content creators the opportunity to license their work for reuse under more flexible copyright terms. Visit the Creative Commons website to learn more. The copyright flexibility that Creative Commons provides makes it easier for creators and internet users to share content and knowledge. Some of the organizations that work closely with Creative Commons include