Works in the public domain are no longer under copyright, whether because they were created by a governmental agency, the author has released them into the public domain using Creative Commons Public Domain Dedication or the copyright term has expired. Works in the public domain are no longer owned by anyone - they belong to the population at large.
It is worth noting that there is the potential for derivative works of public domain material (among other types of modifications to public domain materials) to be protected under copyright.
How long a copyright lasts, and whether an item is still protected by copyright or if it has entered the public domain, depends on several factors, including when it was published, if it is unpublished, when the author died and if the copyright renewal met all the necessary formalities. Currently, copyright is life of the author +70 years.
For a chart on copyright terms, see "Copyright Term and the Public Domain in the United States." For determining if a work created in the United States between January 1, 1925-December 31, 1977 is in the public domain, see the "Is It in the Public Domain?" Handbook and accompanying Flowcharts (please note, as of 2020, public domain works are those published before 1925 rather than 1923).
There are four common ways that works end up in the public domain:
In general, the copyright term for a work created in the United States after 1977 (that is not a work made for hire) is the life of the author plus 70 years. All works published in the United States before 1925 are in the public domain. Works published between 1925 and 1978 may be protected by copyright if they were published with notice and the copyright was renewed.
This is no longer required for works created after 1978 but for these earlier works, lack of renewal means the copyright has expired.
To place an item in the public domain intentionally, a creator of a work would need to state that intent explicitly (as with the Creative Commons Public Domain Dedication). If no such statement has been made, you should assume the work is protected by copyright.
Copyright will not protect the titles of a book or movie, nor will it protect short phrases such as "Make my day" (though trademark protection may apply). Copyright protection also doesn't cover facts, ideas or theories, although it may protect the expression of those ideas.
Any work created by a US government employee or officer is in the public domain, provided that the work is created in that person's official capacity. This applies only to US government works, not the works of other national or state governments.