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:
NoDerivatives (ND): You let others use your work, but they cannot creative derivative works from it.
NonCommercial (NC): You let others use your work as long as it is not for commercial purposes.
ShareAlike (SA): You let others use your work as long as they distribute any derivative works they create from it on the same terms as yours.
On the other hand, if you publish a work under a CC license that does not include the NonCommercial provision, you should assume that someone might try to profit from the work, as cited in this recent example (and see follow-up blog entry).