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. For more information, see Wikipedia's entry on the public domain: http://en.wikipedia.org/wiki/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 the public domain page on their copyright site: http://www.copyright.gov/pr/pdomain.html