When a work is no longer protected by intellectual property rights (e.g., copyright, trademark) it is considered a part of the public domain. No single person owns the public domain; as the name implies, it is available to everyone. This means that you do not need permission to use, adapt, or distribute these works.
There are four ways that works commonly rise into the public domain:
- The copyright has expired. As of 2019, copyright has expired for all works published in the United States before 1924. In other words, if the work was published in the U.S. before January 1, 1924, you are free to use it in the U.S. without permission. Cornell University Libraries provides a commonly cited guide to Copyright Terms and the Public Domain in the United States.
- The copyright owner failed to follow copyright renewal rules. Many works published before 1964 fell into the public domain when their copyrights were not renewed, as required at the time. To determine if a work of interest to you might fall under this class, you will need to search the Copyright Office records. Stanford University Libraries provide an excellent, detailed guide on Searching the Copyright Office and Library of Congress Records.
- The copyright owner deliberately places it in the public domain. Copyright protection is automatic for any original, fixed work. Releasing that protection is actually somewhat difficult and the legal implication of relying on dedication statements hasn't been tested in court. Some authors use the Creative Commons CC0 license fully waive copyright.
- Copyright law does not protect this type of work. Remember, everything doesn't have copyright protection: facts, words or phrases, processes, etc. Another large exception is work created by a US federal employee or officer if that work was created in that person's official capacity. So words from a Presidential speech would be in the public domain. Keep in mind, though, that some publications from the federal government may include additional work that is covered by copyright, such as images licensed from a photographer. Pay attention to notices on and within any document. Also, this does not automatically extend to state or local government works.
Look at all of these methods in depth at Welcome to the Public Domain from the Stanford University Libraries.