What is a Wiki?

I'm glad you asked ;-) Let's quote another Wiki - the Wikipedia:

A wiki (<wee-kee> or also <wick-ey>, also why-kie. according to Ward Cunningham) is a group of Web pages that allows users to add content, as on an Internet forum, but also permits others (often completely unrestricted) to edit the content. The term wiki also refers to the collaborative software (wiki engine) used to create such a website.

In essence, the wiki is merely a vast simplification of the process of creating HTML pages, and thus is an effective way to exchange information through collaborative effort.

So in short: It's a webbased way to collaboratly edit informational pages. You really should read the Wikipedia Article for more details.