From The Art and Popular Culture Encyclopedia
Free content, or free information, is any kind of functional work, artwork, or other creative content having no significant legal restriction relative to people's freedom to use, redistribute, and produce modified versions of and works derived from the content.
Free content encompasses all works in the public domain and also those copyrighted works whose licenses honor and uphold the freedoms mentioned above. Because the law by default grants copyright holders monopolistic control over their creations, copyrighted content must be explicitly declared free, usually by the referencing or inclusion of licensing statements from within the work.
(Work in the public domain cannot be licensed because, by definition, its copyright has expired or has been relinquished. However, such a work is still considered free content, because it may be used for any purpose whatsoever — except, naturally, being re-copyrighted.)