Public Domain refers to creative works that are not protected under copyright law. Permission is not needed to use, modify or perform these creative works.They are available to all but owned by no one.
Some works, by their nature, are not protected by copyright law. These include works not in a tangible form (i.e. improvisational speeches, unrecorded works etc.). Works consisting entirely of common information such as calendars are also not protected.
Federal Government publications are typically in the public domain. Exceptions may occur if a government publication was produced by an outside contractor rather than a government agency or employee working in their official capacity. State and local government documents are not guaranteed to be in the public domain.
A third area of public domain covers those materials in which copyright protection no longer applies. The reasons include the copyright term limit expired or the individual did not renew the copyright. While copyright holders have several protections to ensure they can profit from their creations, the law recognizes that these protections should not be in perpetuity. When the copyright term for a work expires, it enters into the public domain so that others can use, recreate or adapt a work without having to seek permission from the creator.
Suggested Resource: Peter B. Hirtle's chart on copyright and items in the public domain is a useful place to start to determine whether certain material is in the public domain.