What is the difference between CC0 and the Public Domain Mark ("PDM")?
CC0 and PDM differ in important respects and have distinct purposes. CC0 is intended for use only by authors or holders of copyright and related or neighboring rights (including sui generis database rights), in connection with works that are still subject to those rights in one or more jurisdictions. PDM, on the other hand, can be used by anyone, and is intended for use with works that are already free of known copyright restrictions throughout the world. The tools also differ in terms of their effect when applied to a work. CC0 is legally operative in the sense that when it is applied, it changes the copyright status of the work, effectively relinquishing all copyright and related or neighboring rights worldwide. PDM is not legally operative in any respect – it is intended to function as a label, marking a work that is already free of known copyright restrictions.
Unlike Public Domain, the WTFPL is actually a license, and confers rights to the user of the software as such, whereas Public Domain doesn't confer any rights, but is merely an acknowledgement that no rights are asserted. That's why a license is always preferable to Public Domain; a license explicitly gives permission.
The public domain is a bit hard to define formally, as it varies per jurisdiction and there are even jurisdictions that don't recognize a creator's right to dedicate her works to the public domain. This is exactly the problem CC0 is designed to solve:
CC0 helps solve this problem by giving 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 adapted to the laws of any particular legal jurisdiction, similar to many open source software licenses. And while no tool, not even CC0, can guarantee a complete relinquishment of all copyright and database rights 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 and database systems around the world.
CC0 is a license that is equivalent to dedicating your works to the public domain, but it also takes care of the various legal issues and ambiguities of the public domain by simply being a license. The same, more or less, holds true for WFTPL, it's an extremely permissive license you can apply to your works and not worry about the nuances of the public domain.