From the organization:
Creative Commons is a global nonprofit organization that enables sharing and reuse of creativity and knowledge through the provision of free legal tools. Our legal tools help those who want to encourage reuse of their works by offering them for use under generous, standardized terms; those who want to make creative uses of works; and those who want to benefit from this symbiosis. Our vision is to help others realize the full potential of the internet. CC has affiliates all over the world who help ensure our licenses work internationally and who raise awareness of our work.
Although Creative Commons is best known for its licenses, our work extends beyond just providing copyright licenses. CC offers other legal and technical tools that also facilitate sharing and discovery of creative works, such as CC0, a public domain dedication for rights holders who wish to put their work into the public domain before the expiration of copyright, and the Public Domain Mark, a tool for marking a work that is in the worldwide public domain. Creative Commons licenses and tools were designed specifically to work with the web, which makes content that is offered under their terms easy to search for, discover, and use.
The sites listed below represent just a small sample of places you can find content licensed through the Creative Commons.