Haven't written this page yet, but will soon! In the meantime, here's at least a link to this excellent chart from Cornell University:
Copyright Term and Public Domain in the United States
Public Domain
Status that denotes ownership of a work has passed from an individual or group of individuals to the public, making the work freely available to use. In the United States, copyrighted works enter the public domain after specified terms. Many federal government documents are also part of the public domain.
Creative Commons
U.S. copyright statute was not designed with the digital world in mind. In 2001, Creative Commons was formed to offer an alternative way to license works that allowed creators to set terms of use related to attribution, commercialization, the creation of derivative works, and the public domain. The flexibility of CC licenses was intended to foster creativity and leverage the fast sharing world of the internet.