Definition: The public domain is generally defined as consisting of works that are either ineligible for copyright protection or with expired copyrights. Public domain refers to the total absence of copyright protection for work.
No permission is needed to copy or use public domain works. Public domain material is available for anyone to use for any purpose, private and commercial.
For the detailed table of public domain dates, see: http://copyright.cornell.edu/resources/publicdomain.cfm