Works that do not have a copyright are in the Public Domain. Works in the public domain are owned by “the public” and can be used without restriction, and cannot be owned privately or “re-copyrighted.”
NOTE: Public Domain does not mean publicly available. Even though you might find it on the internet or in use by other authors its does not mean that it is in the public domain. Furthermore a painting may be in the public domain, but a museum that owns it retains property rights over it an can charge admission to see it. Similarly you may have to pay for physical copies of things like photographs from the Library of Congress or other non-profit institution
In addition to any work whose copyright expired, there are other works in the public domain:
In general, works created by the Federal Government cannot be copyrighted. This is a statutory provision of the copyright law that specifies that government publications are automatically in the public domain.
Remember, however, that works not eligible for copyright protection may be protected as trademarks, or service marks. The CIA logo, for instance, is protected by special statute and cannot be used without permission.2 Similar protections exist for military branches and unit patches and other government agencies.
Although works produced by the Federal Government are not eligible for copyright the government can retain the copyright of works transferred to it. Also, the Federal Government sometimes produces works that include materials generated under contract by non-governmental employees.
For instance, the Smithsonian Folkways sound recording series is protected by copyright and licensed by the artist or a recording company to the Smithsonian and therefore, are not in public domain. Also, some government publications contain photographs that were taken by non-government contractors and are similarly not in public domain, while the text is.
Furthermore, items on Government websites may be protected by Copyright and provided by license or other agreement.
Copyright of works created by state and local governments vary by state. In Maryland most government publications are in the public domain.
There are exceptions like:
Logos, slogans and other symbols may be protected as service marks. Oakland County in Michigan, for instance, has registered its official seal as a service mark.
For more information about Maryland and other states see Harvard University's guide to copyright for State-government produced works.