Any time you pay for media, you’re actually buying a license to use it and that license usually has limitations: for instance, buying a song gives you the right to listen to it but not to use it as background music in a movie.
Creative Commons licenses let you access and use media for free under certain conditions.
an Attribution license means that you must give credit to the work’s creator;
under a No Derivative Works license you are not allowed to alter the work when you use it;
a Noncommercial license lets you use something as long as you’re not charging money for what you’re creating; and
a Share-Alike license means you have to release any product that you use something in under the same Creative Commons license.