Public DomainA license which explicitly revokes any claims of copyright on a work. You can do anything you want with public domain works, including modify them and claim them as your own, re-release them as part of another work (this makes PD compatible with all other licenses), and change the license on your derived works. There is obviously no restriction on selling public domain works, but you'd probably only be able to sell them for the actual cost of the media and distribution.
In addition to works explicitly placed in the public domain, all copyrighted works automatically pass into the public domain after some time (this is why you always see the year on a copyright notice). The duration of the copyright keeps being extended, however. For more info on this and other public domain vs. copyright issues, see the link(s) below.