Lawrence Lessig, a law professor at Harvard, founded the Creative Commons in 2001. This non-profit organization supports authors who apply Creative Commons licenses to the works they create. While materials published under any Creative Commons license (as of 2022 there are seven different licenses) are available at no cost, they are still copyrighted, but—depending on the license—others may use the materials in various ways. The different Creative Commons licenses are defined by variations on three aspects of the work.
All Creative Commons licenses, except for public domain in which the creator places the work in the same class as works in which the copy right protection has expired, require the creator be attributed. This is identified as BY in the type of license. If you find a useful work published under a CC-BY license, you can make a copy, edit it, then distribute it for others to use, including selling the derived work.
Unless a creator specifies a non-derivative (ND) license, others are allowed to edit the work licensed under a Creative Commons license. If you find a useful work published under a CC-BY-ND license, you can make a copy of it and distribute it, even sell it, but you may not change it.
When licensing a work under a non-commercial (NC) license, other may create derivative works, but they may not use them for commercial purposes. If you find a useful work published under a CC-BY-NC license, you can make a copy of it, edit it, and distribute it along with your edits, but you may not sell it.
When licensing under a share-alike (SA) license, others are allowed to make derivative works, but those must be shared under the same license. If you find a useful work published under a CC-BY-NC-SA license, you can make a copy of it, edit it, and distribute it along with your edits, but you may not sell it, and you must license your derivative with the same license.