Creative Commons is a non profit organisation that allows people to obtain free licensing for their work so they can decide the copyright terms for themselves. Creative Commons licenses look like this:
Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
This particular license means that you can use my work, build on it, but you must credit me, it cannot be used for commercial purposes and you must share it under the same conditions I have shared it. You can find more details about what each license involves here: Licences - Creative Commons Australia.

On the Creative Commons site you can use their licensing tool to stipulate how you want your work to be shared and then it will give you a license based on your choices.

The following videos explain what Creative Commons is and why it is important.

This TED talk by Larry Lessig about how the law chokes creativity is worth watching: