A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone.
Works fall into the public domain for three main reasons:
1. The term of copyright for the work has expired;
2. The author or distributor failed to satisfy statutory formalities to protect the copyright or
3. The work is a product of the U.S. Government.
As a general rule, most works enter the public domain because of their 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.) A smaller group of 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).
More information is available at:
Copyright Slider Tool (A helpful tool to determine is a work is in the public domain or if it is still protected by copyright)