Creative Commons (CC) is an organization that provides access to works for the public to use. Authors can opt to allow others to use their work with a CC license. A work that is CC-licensed will specify the terms of use through a visual notice, such as whether it can be used commercially.
CC licenses allow people to skip the step of contacting the author of a copyrighted work to ask for permission, since the terms of use are displayed with the license. As long as the terms are followed, the author cannot revoke the right of others to use their work.
A detailed breakdown of types of CC licenses, what they mean, and how you can use the works they protect can be found here.
Attribution is required for use of all CC-licensed materials. When attributing a CC-licensed source, always include the following:
This CC license, CC BY-SA, indicates that attribution is required (BY) and that any adaptations must be licensed under the same terms (SA).