"A Wiki or wiki (pronounced "wicky" or "weekee") is a website (or other hypertext document collection) that allows a user to add content, as on an Internet forum, but also allows that content to be edited by any other user." -- from Wikipedia.
So why a World of Warcraft Wiki? As you're going through looking for information, if you see something that's incorrect, you can correct it. If you see any information that's missing, you can add it. This way, the information here should be as up to date as possible, instead of having to wait for a small group to update the information as the game changes. The more people who use the Wiki, the more up to date the information will be.
Click on the Log In button in the upper right to create an account, and check out Help:Editing on some help information on how to get started with editing pages. Use the Sandbox to play around with Wiki editing.