Creative Commons gives you the ability to state how you'll let people use your work -- from "do anything you want to with it," to "do what you want, so long as you're not making money off it," to "do what you want, just give me credit," and a few things in between. (Fr'instance, my Flickr photos are all licensed for non-commercial use, derivative works allowed (but only under a similar license, credit required.) I prefer it to releasing the work into the public domain because I don't want someone taking my work, building on it, and then telling others that they can't do the same thing with whatever they've created with my original work -- Creative Commons lets you keep some control over what people further downstream do with your work. (Not that anyone's riffing off any of my photos, so far as I know, but just in case...)