Public Domain – PDClips.com

Public domain works are works that have no copyright protection and may be used freely for any purpose with the following considerations:

People depicted in the work may still have publicity rights.
Trademarks in the work may still be protected under trademark law.
In the case of composite works like a TV show or motion picture, although the main work may have fallen into the public domain, elements of the work such as the script or music may still have copyright protection.
Even if the work is in the public domain in the United States, it could still have copyright protection in other countries.

Works fall into the public domain for a number of reasons:

On older works, the copyright may have expired or not been renewed. (Typically, all works copyrighted before 1923 have fallen into the public domain.)
The work was specifically placed in the public domain.
The work was created by the United States federal government.
For older works, the work may have fallen into the public domain if it was published without the proper copyright notice.

The Creative Commons organization has developed several marks to indicate that works are in the public domain.

More information on public domain works