Creative Commons License
Creative Commons makes use of four different license conditions - attribution, share-alike, noncommercial, and no derivative works - in varying combinations to create six distinct, and free, usage licenses. Several million pages of web content are currently governed by Creative Commons licenses.
The attribution tag permits others to copy, distribute, display, and perform your copyrighted work - and the derivative works based upon it - but only if they provide you with the credit you request.
"Share-alike" permits others to distribute derivative works but only under a license identical to the license that governs your work. The non-commercial tag lets others copy, distribute, display, and perform your work - and derivative works based upon it - but for noncommercial purposes only.
And "no derivitive works" lets others copy, distribute, display, and perform only verbatim copies of your work, not derivative works based upon it.
Creative Commons: Guilt-Free Use of Other's Work
Read blog entry by Princeton staff.