A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone.
Stanford University Libraries provides a detailed description of public domain with links to freely-available online resources.
Works are in the public domain for four main reasons:
Works published in the United States before 1923 are in the public domain.
Copyright renewal was required for works published between 1923 and 1963. The copyright coordinator can determine whether a copyright was renewed.
Works published before 1977 without the copyright notice © are in the public domain.
Unpublished works pass into the public domain 70 years after the death of the author. As an example for 2015, works created by an author who died prior to 1944 are in the public domain. The estate of a deceased author cannot control the work after this time period.
A work created by a corporation or an unknown author passes into the public domain 95 years after publication.
Copyright Term and the Public Domain in the United States is a detailed listing of the schedule provided by Cornell University.