Works in the public domain are those that are never protected by copyright (like facts or discoveries) or works whose term of protection has ended either because it expired or the owner did not satisfy a previously required formality. Currently, all pre-1928 U.S. works are in the public domain because copyright protection has expired for those works.
From Stanford University's Copyright and Fair Use Guide:
"An important wrinkle to understand about public domain material is that, while each work belongs to the public, collections of public domain works may be protected by copyright. If, for example, someone has collected public domain images in a book or on a website, the collection as a whole may be protectable even though individual images are not. You are free to copy and use individual images but copying and distributing the complete collection may infringe what is known as the “collective works” copyright. Collections of public domain material will be protected if the person who created it has used creativity in the choices and organization of the public domain material."
"A collective work is a compilation in which a number of contributions, constituting separate and independent works in themselves, are assembled into a collective whole. The “authorship” in a collective work comes from the original selection, coordination, and arrangement of the independent works included in the collective work."
The U.S. Copyright Act clearly states that copyright protection in the U.S. is not available for any work of the federal government. The Act states:
105. Subject matter of copyright: United States Government works
Copyright protection under this title is not available for any work of the United States Government, but the United States Government is not precluded from receiving and holding copyrights transferred to it by assignment, bequest, or otherwise.
U.S. government works are in the public domain (i.e., not protected by the U.S. Copyright Act). You can freely use them (in a copyright sense) without obtaining permission or paying a copyright fee. You can even edit, adapt and republish these government works without permission.
Public domain works produced by the government include:
Always keep in mind, however, that this general rule pertaining to U.S. government works doesn't permit you to use all such works.
To be in the public domain, U.S. government works must be created by a federal government employee as part of their official duties. This excludes works created by contractors, freelancers and certain people who work with the U.S. government but aren't considered government employees for copyright purposes.