When a work is in the public domain, it is free for use by anyone for any purpose without restriction under copyright law. Public domain is the purest form of open/free, since no one owns or controls the material in any way.
Works that are in the public domain in one legal jurisdiction are not necessarily in the public domain worldwide. Copyright laws differ from jurisdiction to jurisdiction, both in duration of protection and what constitutes copyrightable subject matter. For example a US Government work clearly in the public domain in the United States may or may not be free of copyright restrictions and in the public domain in other jurisdiction. At present, one of the only ways to be certain that a particular work is in the public domain worldwide is to see if the copyright holder has dedicated all rights to the work to the public domain by using CC0.
Creative Commons licenses do not affect the status of a work that is in the public domain under applicable law, because our licenses only apply to works that are protected by copyright. For more information, see our Licensing Guide to what you should know before you license a work using CC licenses.
Cornell University has also provided a handy table about copyright term and the public domain in the United States.
See http://creativecommons.org/publicdomain for more information on CC's public domain tools.
Public domain content in practice is any content without copyright. It may be freed from restrictions by the copyright, or the copyright may lapse after a certain time.
The details in practice vary between countries. More information about public domain can be found at Wikipedia's article on the public domain.
Appropedia's Public Domain Search was started in late 2007 when it was discovered there was no effective public available. This operates on a completely different method to searches using the Creative Commons marks. It uses a manually maintained index of known sites to be public domain - thus it is does not yield 100% public results, and content must be checked to confirm public domain status. Reliability is expected to improve and feedback by users is encouraged .
As CC public domain tools become widely used, searches based on the CC marks would be expected to a take over from Appropedia's Public Domain Search.