The following information has been copied from the Creative Common website.
The Creative Commons copyright licenses and tools forge a balance inside the traditional “all rights reserved” setting that copyright law creates. Our tools give everyone from individual creators to large companies and institutions a simple, standardized way to grant copyright permissions to their creative work. The combination of our tools and our users is a vast and growing digital commons, a pool of content that can be copied, distributed, edited, remixed, and built upon, all within the boundaries of copyright law.
All Creative Commons licenses have many important features in common. Every license helps creators — we call them licensors if they use our tools — retain copyright while allowing others to copy, distribute, and make some uses of their work — at least non-commercially. Every Creative Commons license also ensures licensors get the credit for their work they deserve. Every Creative Commons license works around the world and lasts as long as applicable copyright lasts (because they are built on copyright). These common features serve as the baseline, on top of which licensors can choose to grant additional permissions when deciding how they want their work to be used.
A Creative Commons licensor answers a few simple questions on the path to choosing a license — first, do I want to allow commercial use or not, and then second, do I want to allow derivative works or not? If a licensor decides to allow derivative works, she may also choose to require that anyone who uses the work — we call them licensees — to make that new work available under the same license terms. We call this idea “ShareAlike” and it is one of the mechanisms that (if chosen) helps the digital commons grow over time. ShareAlike is inspired by the GNU General Public License, used by many free and open source software projects.
Our licenses do not affect freedoms that the law grants to users of creative works otherwise protected by copyright, such as exceptions and limitations to copyright law like fair dealing. Creative Commons licenses require licensees to get permission to do any of the things with a work that the law reserves exclusively to a licensor and that the license does not expressly allow. Licensees must credit the licensor, keep copyright notices intact on all copies of the work, and link to the license from copies of the work. Licensees cannot use technological measures to restrict access to the work by others.