Creative Commons can be used to allow creators of various types of work and media to voluntarily share their work with others. Sites like Wiki use a creative commons licence to allow people to share and distribute work.
The author of the work can licence people under creative commons to use the work and the licence will state any conditions on using and sharing the work; for example, you may need to attribute the work. When you click on an image on a site like Wikipedia, if licenced under the creative commons licence it will tell you how you can use the work.