Creative Commons is a type of license, which is somewhere between a traditional all-rights-reserved copyright and public domain. There are many variations of CC licenses, and they’re onto the 3.0 version of the licenses, so expect more soon. Generally CC licenses require Attribution, which is to say, you can do stuff with this content, so long as you say where you got it from. Often this is in the form of a hyperlink back to the original author’s website. Flickr popularized this by making CC licenses an option on all their photos. You’ll see that almost all of my photos are CC licensed.
Creative Commons licenses can either allow somebody to make commercial use of your material, or not, at your discretion, assuming you’re the one who created the license. Independently, you can allow anybody to modify, adapt, or remix the content. Or not. Or you can allow modification so long as the modified content shares the same license, a so-called “Share Alike” license. Here’s a nice page that shows you the options and allows you to pick a license appropriate for your material.
A CC attribution license is in many senses more realistic in the modern world than an all-rights-reserved license. It is practically impossible to stop people from using or distributing your work. The all-rights-reserved license is a threat to take legal action to prevent somebody from using your work. But suing somebody is such a hassle that it almost never happens for personal content. Asking somebody to put a link to your website is a pretty reasonable thing and easy to accomplish. An all rights reserved copyright is for most individuals a bluff.
CC content is also easier to use. Negotiating terms of licensing under a traditional copyright is daunting. It necessarily requires a back and forth with the author and probably a whole lot. The underlying mindset is that content costs money, so if you’re going to use my content, then you’re going to sell it and I deserve some of that money. As the music industry is slowly, painfully learning, in modern times this model doesn’t work so well. Access to information is generally free, and those who are making money here are doing so by providing value-added services on top of merely distributing the information. (Think ads by Google, or concerts for music.) With a CC license the terms of use of the license are right there. No need to negotiate. Just follow the attribution instructions and do what you will. Instead of requiring negotiation and payment in the traditional economy, this is payment in the nascent reputation economy.