Creative Commons licenses work within copyright law to allow creators to authorize use of their work with some rights reserved.
All licenses include attribution to the licensor and can also include no derivatives, where the work must remain unchanged, and/or limiting derivatives to non-commercial use and/or Share-Alike (derivatives must have the same license).
Copyleft and Share Alike
Copyleft is an alternative to releasing works into the public domain - works are still free and open to the public but derivatives of it must be licensed in the same way.
The Share Alike option offered by Creative Commons ensures that future uses of a work continue to be licensed with the same Creative Commons license.
The GNU General Public Licenses work much in the same way for software. In general, using this license will require that all of the future improvements to the software be free.
