A person using CC0 (called the “affirmer” in the legal code) waives all of his or her copyright and neighboring and related rights in a work, to the fullest extent permitted by law. If the waiver isn’t effective for any reason, then CC0 acts as a license from the affirmer granting the public an unconditional, irrevocable, non exclusive, royalty free license to use the work for any purpose."
Public domain mark FAQ from the CC people.
You'll probably want to look at http://wiki.creativecommons.org/CC0_FAQ which is the disclaimer from copyright (CC0) FAQ.
An excerpt from the later:
"How does it work?
A person using CC0 (called the “affirmer” in the legal code) waives all of his or her copyright and neighboring and related rights in a work, to the fullest extent permitted by law. If the waiver isn’t effective for any reason, then CC0 acts as a license from the affirmer granting the public an unconditional, irrevocable, non exclusive, royalty free license to use the work for any purpose."