© & ™ All rights reserved, Wikimedia Foundation, Inc.
-- From "Introduction" of Wikipedia: The Missing Manual
One of the five main pillars of Wikipedia (which we'll touch on later) is that Wikipedia is free content than anyone can use, remix, and redistribute. All text contributions on Wikipedia, therefore, are licensed with a CC BY-SA license. Non-text media are made available under a variety of licenses, including Creative Commons, GNU General Public License, and public domain. Non-free images are, when necessary, made available at low-resolution under a fair use rationale.
A great introduction to Creative Commons licenses for students or anyone unfamiiar with open licensing.