The public domain, according to Duke’s Center for the Study of the Public Domain, “is the realm of material—ideas, images, sounds, discoveries, facts, texts—that is unprotected by intellectual property rights and free for all to use or build upon.” In the United States, anything that was published before 1923 is in the public domain. Works published between 1923 and 1963 may be in the pubic domain, if they were published without notice (the symbol (c) with a date and name), or if the original copyright was not renewed after the first term of 28 years. It is often difficult to be certain about this, although the database of renewal records made available by Stanford University is a big help. Government works — works created by government employees (but not necessarily independent contractors working for the government) — are also in the public domain because the copyright law does not allow an initial claim of protection in such works. Works published with a Creative Commons license may also be in the public domain, although usually they are partially protected by copyright but available for non-profit reuse. Unpublished works are in the public domain if the author died over 70 years ago. It is important to note that all of these rules have some additional complexities; this chart by Peter Hirtle is very useful for sorting out the intricacies of copyright terms.
Most importantly, facts and ideas are in the public domain, since copyright only protects expression. Patents, however, do protect ideas, so the idea of a patented invention is not free for others to use without a license, while ideas contained in copyrighted expression are.