U. S. copyright law, Title 17 of the U.S. Code (https://uscode.house.gov/) provides legal protection to the creator of a work that is fixed in a tangible medium of expression. It protects literary, dramatic, musical, and artistic works. "Fixed in a tangible medium of expression" means that it has been captured in some format that is at least semi-permanent and may be accessed again, including written, drawn, photographed, filmed or recorded. It can be a hard copy or a digital file. Under current U.S. law, all such works are automatically copyrighted and do not require that they be registered or published with a copyright notice. Copyright reserves six rights for the author: reproduction, distribution, creation of derivatives, public performance, public display, and for public performance of sound recordings via digital audio. Copyright protection lasts for life of the author plus 70 years; or in the case of works for hire, unknown authorship, or corporate authorship, 95 years from date of creation (if known) or 120 years if date is unknown.
Note that each nation has its own copyright laws, and the above information is for the United States only. The United States is a member party to several treaties that respect the copyright laws of other nations.
Open licenses overlay copyright by allowing the copyright holder to give up some or all of their exclusive rights. This is most often done using Creative Commons licenses, or in the case of software, the GNU General Public License.
Educators are familiar with the doctrine of Fair Use, which allows exceptions for the use of copyrighted materials in certain situations. Developed by the Center for Media & Social Impact, which is based at American University's School of Communication, the Code of Best Practices in Fair Use for Open Educational Resources provides fair use guidelines for authors, adapters & adopters of openly licensed educational materials.
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.
Attribution
CC BY
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.
Attribution-ShareAlike
CC BY-SA
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.
Attribution-NoDerivs
CC BY-ND
This allows redistribution either commercially or not, but the original may not be altered in any way (No Derivatives) and the original creator must be attributed.
Attribution-NonCommercial
CC BY-NC
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 ND (NoDerivs).
Attribution-NonCommercial-ShareAlike
CC BY-NC-SA
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.
Attribution-NonCommercial-NoDerivs
CC BY-NC-ND
This only allows non-commercial redistribution with attribution.
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.
As of Jan. 1, 2022, the effective date of copyright in the U.S. is 1927. Anything published in the United States prior to that is in the public domain and may be copied, distributed, or modified. For items published in 1927 or after, it is trickier to determine whether the copyright has expired or not. Also, items that were never published or were originally published outside the U.S. have different rules governing copyright and public domain. 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. Every year on January 1, the effective date rolls forward by one year, so on 1/1/2023, things published in 1927 will move into the public domain; on 1/1/2024, things published in 1928 will move into the public domain, etc.
All content now produced is automatically 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.
There are Creative Commons tools to designate a work in the Public Domain. The CC0 license indicates that the creator relinquishes all rights to the work under copyright laws in all jurisdictions. Remember that copyright laws vary country-by country; this tool (CC0) will be universally applied to the work. For more information on CC0 see: Public Domain- Creative Commons.
If you have identified a work as in the Public Domain, you can mark it with the designation 
Be very cautious in use of this Public Domain mark unless you are placing it on your own work. Copyright laws vary widely across the world and copyright is often less clear on unpublished works or works that were not commercially published. See Public Domain Mark - Creative Commons for more information.
