Creative Commons is a "nonprofit organization that aims to increase sharing and improve collaboration" through provision of simple licensing options. Creative Commons provides creators with licensing to encourage the sharing and distribution of their work. Although the author's work remains copyrighted, a Creative Commons license allows others to copy, share and reuse the work with limited or no restrictions.
Learn more about Creative Commons on their website.
There are two ways to identify if a work has a Creative Commons license:
Creative Commons Search allows you to search for images and other content that has a creative commons license. Note that Creative Commons claims no control over the results that are returned and suggests users always verify that the work is actually under a CC license by following the link.
Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through content under each type of license.