I receive the Sitepoint newsletter, along with several thousand other people. One of their featured articles is this:
http://www.sitepoint.com/article/1241What is a Wiki? By Nathan Matias
If this is a question you've asked yourself, you're not alone! Nate clears up the confusion with this in-depth look at the wiki. Starting at the very beginning, he explores wiki history, functionality, software, and more.
It includes several links to the English Wikipedia (including one to JTDirl's contribution list!).
--Catherine