What is Creative Commons?
Creative Commons [creativecommons.org] is a project that helps people license their copyrighted work so that others can use it for free.
Creative Commons takes care of writing licenses that are legally valid around the world and hosting them so they remain available via permanent URLs.
There are many kinds of Creative Commons licenses. They're typically referred to by abbreviations starting with CC (for Creative Commons), such as CC BY, CC BY-SA, CC BY-NC-ND, and so on.
There are also multiple versions of CC licenses, as they're updated to reflect newly found legal issues, so you have CC BY-SA 2.0, CC BY-SA 3.0, and so on.