CC0 is one of the intellectual property rights designations created by the Creative Commons organization. Here is what Creative Commons have to say about CC0:

CC0

“No Rights Reserved”

“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.

In contrast to CC’s licenses that allow copyright holders to choose from a range of permissions while retaining their copyright, CC0 empowers yet another choice altogether – the choice to opt out of copyright and database protection, and the exclusive rights automatically granted to creators – the “no rights reserved” alternative to our licenses.”

The Creative Commons organization exists to solve the problems associated with legal intricacies governing how copyrights are protected from country to country. Prior to the advent of Creative Commons and CC0 licensing, it was complex and expensive for a copyright owner to release his or her work to the public. CC0 licensing makes it easy for owners to release their work for others to use. 

As a result of the growing number of out of copyright works released officially with CC0 designations many high quality digital representations of CC0 work can now be easily found. One note of caution should be observed. Many of the sites hosting high quality CC0 imagery have gone to considerable trouble to produce the best possible quality digitizations. These images are often cleaned or digitally enhanced by skilled digital artists. The web sites presenting these images are works of art in themselves and represent substantial investment of time and money on the part of the web providers. Often the web sites publish rules which limit to some extent your freedom to use the images they have found, cleaned up and carefully presented. Understandably we consumers of CC0 imagery can not simply download another groups images a build a website doing essentially the same thing with the images as the business from which the images were downloaded. If you have doubts about the legal and ethical position of your intended use when downloading from CC0 aggregator, it is best to ask the web site owners if your intended use is free of infringement on their preexisting web business.  I have found this approach to be greatly appreciated by the web site owners, and I have found them to be supportive of creative endeavors which do not compete directly with them.