If a work is in the public domain, that means that either the copyright has expired, or the creator of the work has forfeited all property rights. Publications created by the US government are also in the public domain. Anyone can use a public domain work without obtaining permission from the creator.
Copyright law has changed a lot over the years. Currently, works fall into the public domain 70 years after the death of the copyright holder. For more details about copyright terms, check out this helpful guide created by the Cornell University Library.
Any work assigned a Creative Commons public domain license by its creator.
All works published in the United States before 1923.
All works published in the United States between 1923 and 1977 without a copyright notice.
All works published in the United States between 1923 and 1963 that did not obtain a copyright renewal.
All works published in the United States between 1978 and March 1, 1989 that did not obtain copyright renewal five years after creation.
If you are unsure about whether or not a work is in the public domain, check this guide created by the US Copyright Office to help you determine the status.
Curious about how public domain law has changed over the years? This video explains how Disney helped to change copyright law.