A public domain work is a creative work that is not protected by copyright and which can be used freely by everyone.
More information available at:
University of Minnesota Libraries
Stanford University Libraries
Works fall into the public domain for three main reasons:
1. When the copyright work term expires;
2. When the author doesn't follow the proper formalities for copyright; and when
3. It is U.S. government work.
Many public domain resources can be found on the web.