Copyright
Copyright is based on a complicated collection of laws, that vary in different countries. Every country has their own legislation. On top of that big corporations (such as Disney and other Hollywood giants) are trying to get to a "uniform" copyright legislation all over the world. This wouldn't be bad weren't it for the fact that it means that copyright legislation is becoming stronger and longer and giving copyright holders much more power. In the end Mickey Mouse (1928) will never enter the public domain. The same public domain that Disney has taken countless stories from to turn into profit.
Fortunately Creative Commons, founded by Lawrence Lessig, an American professor at Harvard Law School and 2 other experts in law, is there to make our lives easier again. The licenses that Creative Commons created allow creators of copyrightable works to give freedom to other people without them specifically having to contact the creator.
Creative Commons licenses allow creators to give certain freedoms to people while retaining certain rights.
There are 4 different clauses, from which licenses can be built:
Attribution (by) Licensees may copy, distribute, display and perform the work and make derivative works based on it only if they give the author or licensor the credits in the manner specified by these.
Noncommercial (nc) Licensees may copy, distribute, display, and perform the work and make derivative works based on it only for noncommercial purposes.
No Derivative Works (nd) Licensees may copy, distribute, display and perform only verbatim copies of the work, not derivative works based on it.
Share-alike (sa) Licensees may distribute derivative works only under a license identical to the license that governs the original work.
There are several different Creative Commons licenses, but on all of the licenses it's required to attribute the creator (the so called BY clause).