When a work is in the public domain, it is free for use by anyone for any purpose. Public domain is the purest form of open/free, since no one owns owns or controls the material in any way. In other words, it is free of any license. For more information, see Wikipedia's entry on public domain.
When does a work enter the public domain?
This varies by country. To see when a work in the United States enters the public domain, see copyright.gov's public domain page.
Cornell University has also provided a handy table about different kinds of works: http://www.copyright.cornell.edu/public_domain/