Creative Commons is a nonprofit organization that aims to promote the sharing and reuse of creative works. To this end, they have created a series of licenses that authors can attach to their works to establish rules for how their works can be used and/or shared. All of the licenses require that the creator of the work be attributed, but some of the licenses impose additional requirements:
ShareAlike (SA): You let others to use your work as long as they distribute any derivative works they create from it on the same terms.
NonCommercial (NC): You let others use your work as long as it is not for commercial purposes
NoDerivatives (ND): You let others use your work, but they cannot creative derivative works from it.