Is "Public Domain" the same as "CC-0"?

When a work is in the public domain, that means it is not under copyright: it can be used by anyone for anything. The work does not have a license; instead, the author (or whoever would be the copyright holder) usually just states that the work is in the public domain. But because the concept of the "public domain" is defined differently by different nations, and not defined at all in some, there is no way to reliably place a work into the public domain worldwide. This makes it legally ambiguous in international copyright law.

The Creative Commons "No Rights Reserved" (CC-0) instrument is a solution to this problem. CC-0 is a universal method for ceding your rights to a created work. There is no assumption that this can be done under the laws of all nations, but by using CC-0 the author makes their position clear.

For Launchpad's purposes, we treat a project license of "Public Domain" as similar to saying that CC-0 applies to that project, since the intent of the two should always be the same.

Creative Commons calls CC-0 an "instrument" instead of a "license", since it signifies an intent to give up rights. In their words, CC-0 gives "...creators a way to waive all their copyright and related rights in their works to the fullest extent allowed by law. CC0 is a universal instrument that is not ported to any particular legal jurisdiction, similar to many open source software licenses. And while this means that CC0 may not be completely effective at relinquishing all copyright interests in every jurisdiction, we believe it provides the best and most complete alternative for contributing a work to the public domain given the many complex and diverse copyright systems around the world."

See http://creativecommons.org/about/cc0 and http://wiki.creativecommons.org/CC0_FAQ for more information.