There are often references to books and other printed matter that has entered public domain. What this means is that while the printed works were at one time copyrighted and therefore considered the intellectual property of the author and publisher, that is no longer the case. In many cases, the work is no longer in print by the original publisher, the author is deceased, and the copyright was allowed to expire. In effect, the work is no longer owned by a person or entity. When no ownership can be established, the work is considered to be in the public domain.
Along with printed matter, the same general principle applies to early motion pictures that were made before 1922, or were produced by studios that no longer exist. When there is no evidence that someone today is the beneficiary of those works and can reasonably claim ownership of the films, they are considered to be in the public domain. This means that anyone can obtain a copy of the film and reproduce multiple copies for sale without infringing on the rights of anyone.
In addition to works that were once copyrighted but no longer enjoy that status, there are works intentionally created for general public use. Some government documents are an excellent example of this type of public domain product. Unless there are disclaimers to the contrary, government documents are understood to be accessible and usable by everyone, without the need to observe a copyright. Generally, however, it is anticipated that if a section of the document is quoted, the quote will be referenced properly.
As a broad definition, public domain materials are any form of knowledge that is freely available to the general public, and carries no restrictions on the use of the materials. Books, movies, and other forms of printed matter are all common examples of public domain information, but essentially any device that previously enjoyed a copyright but is no longer covered would be considered to be in the public domain.