Read this graphic novel depicting the fair use struggles of a documentary filmmaker. It covers copyright basics and intellectual property issues that creative professionals deal with on a regular basis.
Legal definitions of “public domain” as it relates to intellectual property are:
“A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner.”
'The term “public domain” refers to creative materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it.'
“A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone. The reasons that the work is not protected include: (1) the term of copyright for the work has expired; (2) the author failed to satisfy statutory formalities to perfect the copyright or (3) the work is a work of the U.S. Government."
"CC0 enables scientists, educators, artists and other creators and owners of copyright- or database-protected content to waive those interests in their works and thereby place them as completely as possible in the public domain, so that others may freely build upon, enhance and reuse the works for any purposes without restriction under copyright or database law." -- Creative Commons CC0 "No Rights Reserved" mark
Stanford University Libraries have written an excellent overview that explains the public domain, and closely examines the different ways that materials enter the public domain, and public domain trouble spots.