When we say that something is in the "public domain" we mean that it is free for others to use without obtaining permission from the owner. When a work is noted as being in the Public Domain, it is not covered by copyright, or any other intellectual property rights.
Works created by the United States government are automatically in the Public Domain. As taxpayers, we have already paid for the creation of works by the US government, and therefore own the rights to any work created by its employees. PLEASE NOTE: works by state and local governments are likely protected by copyright.
Works whose copyright has expired. Copyright terms vary depending on the type of work, and the publication status. In the United States, all materials published before 1923 are in the public domain. So, if a user wants to make a copy of an entire book that was published in the United States in 1921, the answer is: Go for it! That particular book is in the public domain.
Works released into the Public Domain by the author. Some authors choose to relinquish their intellectual property rights and publish their works in the Public Domain.
Hathi Trust database: one easy way to check if a particular item is, indeed, in the public domain (especially for books) is to check the HathiTrust database. If the entire item is publicly accessible in the HathiTrust database, the library user is free to download and print that item.