Copyright law is different in different countries. What is in the public domain in the United States might not be in the public domain in other countries, and vice versa.
Public Domain -- the legal status of works that are not protected by copyright, either because their creators intentionally (or unintentionally) relinquished copyright protection, or because copyright protection has expired. Anyone is free to use works that are in the public domain without permission.
Copyright -- the name for a set of exclusive rights to use intellectual works, typically these are retained by the creator of the work, or transferred to a publisher.
Open Access -- a term identifying works that are freely available to the public, usually distributed via the World Wide Web -- they may or may not also be in the public domain.
Creative Commons -- a set of predetermined licenses that copyright owners can use to automatically allow people to make use of copyright-protected works in specified ways under specified conditions.