The Creative Commons Blog identifies the public domain as "The space where no intellectual property rights exist." Works can enter the public domain in two ways: when the copyright expires or when the author places a work in the public domain. To determine if copyright has expired, see the links listed below. If the work was published the the U.S. before January 1st, 1924 you are free to use it in the U.S without permission.
The U.S. Copyright Office identifies orphan works as "works that, although their author cannot be identified, are protected by copyright."