Public Domain refers to works that are not copyrighted.  There are several reasons why a work might be in the public domain.

• The work is old enough that the copyright has expired. Works published before 1924 are the public domain in the United States.
• The creator chose not to copyright the work. Many works produced by the U.S. government are born in the public domain.
• The work is not original enough to be copyrighted.  Ideas and facts are not protected by copyright law.