CREATIVE COMMONS? OPEN SOURCE? PUBLIC DOMAIN? COPYRIGHT? COPYLEFT?
Creative Commons: Creative Commons (CC) is a non-profit organisation which facilitates more flexible copyright usage for the internet. Anyone can release their work under Creative Commons, with differing levels of restriction. There is a chart explaining the different licenses at the bottom of this section.
Open Source: Put simply, programming code that can be read, viewed, modified, and distributed, by anyone who desires. Moodle, OpenOffice & Gimp are examples of open source software programs. (see block on right for nifty download links)
Public Domain: Non-copyrighted material which may be used without violating copyright restrictions. Also refers to things like Books and Film which become public domain after their copyright expires.
Copyleft: Not a joke, it's the opposite of Copyright and therefore applies to open source/public domain music.
This chart, explaining the different icons used in creative commons licensing, comes from squidoo.com (link on the right)