Creative Commons licensing is one of several public copyright licenses that enables the distribution, reuse, and expansion of a copyrighted "work" under terms dictated by the creator. Many OERs are licensed under Creative Commons licensing.
Creative Commons offers six different license types, which are all combinations of four different attributes:
The CC BY license is the most common license you will see, and the most accommodating. It allows for distribution, remixing, and adaptation of a work, even commercially, as long as you credit the original creation.
This license allows you to remix, adapt, and build upon a work even for commercial purposes, as long as you credit the creator and license your new creations under the identical terms. All new works based on yours will carry the same CC-BY-SA license, so any derivatives will also allow commercial use.
This license allows you to reuse the work for any purpose, including commercially, as long as you use the work in its original form, and credit the creator. Remixing or adapting the work is not permitted by this license.
This license allows you to remix, adapt, and build upon a work non-commercially.
This license allows you to remix, adapt, and build upon the work non-commercially, as long as you credit the creator and license your new creation under the same terms.
This is the most restrictive license, only allowing you to download the work and share it as long as you use the work for noncommercial purposes, use the work as is, and credit the original creator of the work.