The doctine of “fair use" in U.S. Copyright law lists of the various purposes for which the reproduction of a particular work may be considered “fair,” such as criticism, comment, news reporting, teaching, scholarship, and research.
A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone.
The reasons that the work is not protected include:
(1) the author failed to satisfy statutory formalities to perfect the copyright
or
(2) the work is a work of the U.S. Government.