Wikipedia Reference Information
Public domain comprises the body of knowledge and innovation (especially creative works such as writing, art, music, and inventions) in relation to which no person or other legal entity can establish or maintain proprietary interests within a particular legal jurisdiction. This body of information and creativity is considered to be part of a common cultural and intellectual heritage, which, in general, anyone may use or exploit, whether for commercial or non-commercial purposes. Only about 15 percent of all books are in the public domain, and 10 percent of all books that are still in print.
If an item ("work") is not in the public domain, this may be the result of a proprietary interest such as a copyright, patent, or other sui generis right. The extent to which members of the public may use or exploit the work is limited to the extent of the proprietary interests in the relevant legal jurisdiction. However, when the copyright, patent or other proprietary restrictions expire, the work enters the public domain and may be used by anyone for any purpose.
The complete, up-to-date and editable article about Public Domain can be found at Wikipedia: Public Domain