Creative Commons is an organization & an idea that everyone interested in open source, intellectual property, or creative expression should at least familiarize themselves with. We're all familiar with copyright—'All rights reserved'—but the idea behind CC is 'Some rights reserved'.
As Creative Commons explains itself,
'Creative Commons is a non-profit corporation founded on the notion that some people may not want to exercise all of the intellectual property rights the law affords them. We believe there is an unmet demand for an easy yet reliable way to tell the world 'Some rights reserved' or even 'No rights reserved.' Many people have long since concluded that all-out copyright doesn't help them gain the exposure and widespread distribution they want. Many entrepreneurs and artists have come to prefer relying on innovative business models rather than full-fledged copyright to secure a return on their creative investment. Still others get fulfillment from contributing to and participating in an intellectual commons. For whatever reasons, it is clear that many citizens of the Internet want to share their work—and the power to reuse, modify, and distribute their work—with others on generous terms. Creative Commons intends to help people express this preference for sharing by offering the world a set of licenses on our Website, at no charge.'
You may want to use CC licenses on some of your work, or you may want to use CC licensed work in your own projects, or you may think CC is crazy. No matter which you think, you should learn about CC.
To help people understand Creative Commons, the people behind CC have made the following resources available (all under a CC license, of course):
'How It Works' cartoon
'A Spectrum of Rights' cartoon
'Get Creative' flash animation
I've given a presentation about Digital Rights Management that includes a brief discussion of Creative Commons; the PDF of that presentation is 144 kb and is itself under a CC license.
One of the best ways to learn about CC is to create a license yourself. It's about as easy as can be: you just answer 3 questions, and a license is created for you, in 3 different versions: readable by a normal person, readable by a lawyer ('cause they're not normal people, y'know!), and readable by computers.
And of course, the Creative Commons Web site itself is a treasure trove of information about CC. Take some time and look around the site; pretty much any questions you may have about CC are answered there.
Check out Creative Commons—it's a great idea whose time has come.