Public Domain indicates that an item is NOT protected under copyright law and may be used in any way one wishes. Items for which the copyright has expired or items for which creators have chosen to not retain their rights and have designated as in the public domain may be copied, distributed, modified, displayed, or performed by anyone for any purpose.
The effective date of copyright in the U.S. is 1924. Anything published in the United States prior to that is in the public domain and may be copied, distributed, or modified. For items published after 1924, it is trickier to determine whether the copyright has expired or not. See this chart at Cornell University for a good overview and for help in determining if an item is still protected by copyright. When in doubt, assume it is copyrighted and do not use except in clear fair use situations.
All content now produced is protected by copyright law. In addition to designating a work in the public domain and losing all rights and control of their work, creators have the option to decide if they want to allow exceptions to copyright while retaining some rights through special licensing that designates what others may do with the work. These rights are most often defined through Creative Commons licensing.
Creative Commons (CC) licenses provide the assignment of certain rights to users that are normally held by the copyright holder. CC licenses lay on top of copyright law; works are still protected under the law but the copyright holders have decided to allow certain uses that otherwise would not be allowed.
Content owners may decide how open or how restrictive they wish to be. There are six versions of the Creative Commons licenses that mix the different rights, which are: BY Attribution; SA ShareAlike; ND NoDerivs; and NC NonCommercial.
This allows virtually any use, including commercial, and the original can be altered in any way. The only restriction is that you must acknowledge the original source.
This allows virtually any use, including commercial and any alterations you choose, with attribution to the original. ShareAlike means that any derivatives must be shared under the original license.
This allows redistribution either commercially or not, but the original may not be altered in any way and the original creator must be attributed.
This allows any type of remix or changes but solely for non-commercial use. It must be attributed, but the changed version can be assigned any type of license such as "NoDerivs."
This allows changes or remixing of content for non-commercial use. It must be attributed, and the changed version must be licensed in the same way as the original.
This only allows non-commercial redistribution with attribution.