A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone. Works become part of the public domain because:
As a general rule, most works enter the public domain because of old age. This includes any work published in the United States before 1923. Another large block of works are in the public domain because they were published before 1964 and copyright was not renewed. (Renewal was a requirement for works published before 1978.) Some works fell into the public domain because they were published without copyright notice (copyright notice was necessary for works published in the United States before March 1, 1989).
Generally, works produced by the U.S federal government are in the public domain, but there are exceptions. For instance, if a consultant is hired to produce a work for the U.S. government, then that consultant owns the copyright of that work unless the consultant transfers, in writing, the copyright to the U.S. government.