Copyright is the intellectual property law that protects a creative work from theft or mis-use. It is a legal claim to a work. By default, creators posess the copyright to any original creative work as soon as it is expressed in tangible form. Typical copyright can be thought of as,
All rights reserved.
To use a copyrighted work in any way, one must first obtain permission from its creator or provide clear attribution to the creator if the use might be considered exceptional and adheres to the terms of "fair use."
A license is a permissive layer applied to a copyrighted work that allows a use of that work that would otherwise be prohibited by copyright. A copyright license gives a user specific permissions without needing to request them from the creator or consider the terms of fair use. In short, think:
Some rights reserved.
Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through the creation and distribution of free legal tools.
Creative Commons Licenses are not an alternative to Copyright. They work alongside copyright and enable the modification of copyright terms to fit specific needs. Applying a Creative Commons License to a work can be thought of as changing,
"All rights reserved"
"Some rights reserved."