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 article on public domain.
When does a work enter the public domain?
Cornell University has also provided a handy table about copyright term and the public domain in the United States.