Creative commons
Creative commons is a way for the creators of images or sounds to give permission for other people to use their work.
Attribution[edit]
Attribution means that you have to give credit to the original author. If you use an image or sound that requires attribution, you need to put the author in the credits of your game, or thank them in the README file.
Non-commercial[edit]
If an image or sound is Non-commerical, you may not include it in a game you plan to sell. Non-commercial resources can only be used in free games.
No-Derivatives[edit]
If an image or sound is No-derivatives, you can use it in your game, but you may not change it, No editing, no remixing, no changing. Just use it as it is.
[edit]
If an image or sound is Share-Alike, you can use it and change it only if you allow other people to use and change the result under the same Share-Alike terms. This is similar to GPL