There are several categories that fall within the purview of Public Domain. These are:
What is in Public Domain is very complex, however. For example, works that are expired in the US may still be under copyright in other countries. It is stressed that without a comprehensive search of the publication history of a work and the history of copyright for the format (for example, scores, musical recordings and printed works may legally define Public Domain differently), it should not be assumed that a work is in the Public Domain. Cornell University provides a good resource that outlines the dates, inclusions and exceptions of the Public Domain in the United States.