The legal concept of the public domain as it applies to copyright law should not be confused with the fact that a work may be publicly available, such as information found in books or periodicals, or on the Internet. The public domain comprises all those works that are either no longer protected by copyright or never were.
Essentially, all works first published in the United States prior to 1923 are considered to be in the public domain in the United States, as are works published between 1923 and 1963 on which copyright registrations were not renewed. Materials created since 1989, other than those created by the U.S. federal government, are presumptively protected by copyright. Therefore, the likelihood that materials of greatest interest are in the public domain is low. In addition, you must also consider other forms of legal protection, such as trademark or patent protection, before reusing third-party content.
Public domain materials generally fall into one of four categories:
Also, in rare instances, works may be "dedicated" (donated) to the public domain.
Courtesy of Copyright Clearance Center www.copyright.com