Creative Commons is a non-profit corporation which encourages creators of intellectual property (such as game designs) to allow others to build upon their creations while still optionally retaining some rights, such as the right to profit from the idea.
Some Rights Reserved
Creative Commons offers many possible licenses. Their site has an excellent description of their similarities and differences here.
CC also offers a utility to help users select the appropriate license.
Using CC on a wiki
This image should be placed on any CC-licensed work
Make sure that you link the button to the appropriate license. For example:
You may use a template such as Template:cc donated to apply a CC license to your content.