These categories should be viewed broadly.
Copyright is a form of protection provided by U.S. Congress to the authors of original works of creativity, including literary, dramatic, musical, artistic, and certain other intellectual works. This protection is available to both published and unpublished works. It constitutes the author rights.
Fair Use is a doctrine of the U.S. copyright law that allows limited use of copyrighted works without seeking permission typically for the purposes of criticism, comment, news reporting, teaching, scholarship, and research. When determining whether a use is fair, it is important to take into consideration these four factors:
In the video below, legal experts from the Center for Internet and Society at Stanford University answer questions about fair use.
A public domain work is a creative work that is not protected by copyright and may be freely used by anyone. The reasons that the work is not protected include:
The term of copyright for the work has expired, or
The author failed to satisfy statutory formalities to perfect the copyright, or
The work is a work of the U.S. Government.
Copyright experts have put together charts to help people determine if a work is in the public domain:
When U.S. Works Pass into the Public Domain by Lolly Gassaway
Copyright Term and the Public Domain in the United States by Peter Hirtle
The first day of every year is known as Public Domain Day because the copyright term of certain works expires and they enter the public domain.