Works determined to be in the public domain are not protected by copyright law, belong to "the public", and are free to be copied, distributed, adapted, performed or displayed in public without seeking permission or paying a fee.
Copyright laws of other countries can be very different. A work that is in the public domain in the U.S. is not necessarily in the public domain outside of the U.S., and vice versa. Many countries have agreed to give citizens of other countries the same protection as their own. In that case you would follow the laws of the country in which you intend to use the work to determine whether it is in the public domain. In other words, if you intend to use the work only within the U.S. you should make a public domain determination based on the U.S. copyright law. But if you intend to use material copyrighted in another country, and it is not in the public domain in that country, you must get permission or risk being sued for copyright infringement.
Fore more information on public domain works and non-U.S. copyright law see: