Wikipedia is a free encyclopedia that is being written collaboratively by people very much like you. All of the articles are covered by the GNU Free Documentation License, to ensure that they can remain freely available forever.

If you want to use Wikipedia materials in your own books/articles/web sites or other publications, you can do so, but you have to follow the GFDL, which entails the following:

• Your materials in turn have to be licensed under GFDL
• You must acknowledge the authorship of the article (section 4B) and
• You must provide access to the "transparent copy" of the material (section 4J). (The "transparent copy" of a Wikipedia article is its wiki text.) These two obligations can be fulfilled by providing a conspicuous link back to the home of the article here at wikipedia.org.

http://www.wikipedia.org/wiki/GNU_Free_Documentation_License