Creative Commons (cc) is a non-profit organisation founded by Lawrence Lessig with the aim of facilitating the easy sharing and use of creative works. The premise being that traditional copyright laws are too restrictive in this regard by preventing creatives who would like to make their work more freely available from easily doing so.
It should be noted that an image, photo or an other creative work made available under a creative commons license may be free in that payment for its use isn't required but it is not usually entirely free from restrictions and conditions on the use of the work. The nature of these depends on the particluar CC license the creator of the work chooses to use; there is no single creative commons license but many versions depending on the combination of permissions and restrictions set. The copyright of CC works does still remain with the original creator.
Photos and images offered under Creative Commons Licenses will usually come with the attribution condition. This means the author is happy for the image to be used as long as credit is given, so for instance if the image is used on a blog a credit line and link back to the photographer's own site or Flickr stream for example is required. Often sources offering CC images will provide the details of what is required and any necessary code.
Other common terms used in CC licenses concern the further sharing and distribution of works, the use of works in derivative work and the use of works for commercial purposes. An image made available under a CC license may, therefore, still have rights retained by its author that prevent some uses. It is important to understand the particular license that is being offered in order to avoid infringing an author's copyright; remember CC images are still the copyright of the author and are only free to use within the terms of the CC license they are offering!
Here's a nice video presentation that explains the ideas and benefits of using CC licensing.
So, with that understanding of creative commons licensing, where do you find photos and images to use for free under a CC license?