Also referred to as “No rights reserved” or “not protected by copyright”. This means copyright has either expired or is not applicable to a work. Public domain means that anyone can use the work for any purpose, without attribution or other restriction on reuse. Works published prior to 1923 are in the public domain in the US, for example, as are some works whose copyrights were not renewed prior to 1976 (Night of the Living Dead, etc.).
To the extent copyright may apply, Creative Commons’ CC0 (Read: CC Zero) tool allows for authors to waive all rights, universally in all international jurisdictions, related to copyright so that the work effectively enters the public domain. There are complicated legal reasons why CC Zero is a waiver and not a license, but is applied and expressed in a similar fashion.
Read more about the Public Domain on Wikipedia.
Creative Commons Licenses
Creative Commons was founded in order to create a middle ground between the public domain and traditional copyright. This middle ground helps authors to control the rights automatically granted to them by the law. Creative Commons is not anti-copyright, as CC licenses depend on copyright in order to work. The licenses allow authors to license their work sunder copyright on terms that encourage sharing and reuse of their content in digital media and on the Internet. CC licenses are “public” licenses in the sense that they anyone can use the content in the ways specified by the author. Typically CC licenses are expressed as an image on webpage linking to the CC license on our servers. There is one required component (Attribution) and three options (NonCommercial, NoDeriviatves, and ShareAlike) that can be mixed and that in combination make up CC’s 6 distinct licenses. When a CC license is broken (i.e., the terms of the license are not followed), it is up to the author, not Creative Commons to police the use.
Wikipedia only accepts content licensed under the following two CC licenses:
This license lets others distribute, remix, tweak, and build upon your work, even commercially, as long as they credit you for the original creation. This is the most accommodating of licenses offered, in terms of what others can do with your works licensed under Attribution.
This license lets others remix, tweak, and build upon your work even for commercial reasons, as long as they credit you and license their new creations under the identical terms. This license is often compared to open source software licenses. All new works based on yours will carry the same license, so any derivatives will also allow commercial use.
Traditional copyright or “All Rights Reserved” copyright applies automatically as soon as a work is fixed in a medium. This means everything from your website, to the photos you post are copyrighted by you (or your organization) until 70 years after the author’s death, unless the work is a “work for hire” in which case it is then owned for 90 years after its creation. Excepting fair uses, all re-users of the work must seek permission in the form of a private ad-hoc, negotiated agreement or license with the author or risk infringing on the copyright of the original author.
Read more about traditional copyright on Wikipedia.