Under US law, copyright expires 75 years after the death of the author/creator (renewable once by creator's estate). At that point, works automatically enter the public domain. As a practical matter, most works published in the US before 1926 are now in the public domain and using them does not require permission.
Attention should be paid about post 1926 new editions or revisions of works in public domain that are copyrighted. For example, the music score of a piano sonata by Beethoven who died more than two hundred years ago is in public domain. But post 1926 “edited” scores by publishers such as “Schirmer” or “Peters”, etc. are still copyrighted. The same situation applies to recordings of these works that the copyright holders are the performers and publishers of the recordings not the composers.
Works by the US government or created by its employees as part of their job are in public domain. Note, however, that this does not apply to most works by federal grant recipients or contractors or to works of other governments, including some state and local governments.
The exact date in which a work (specially, audiovisuals and images) is considered in public domain is varied and complicated. Please check the link above (Public Domain) for detailed information.