WhatIsaWiki

A wiki is a collection of interlinked web pages, each of which can be visited and edited by anyone. You can even edit this page: just click "Edit text of this page" below.

So it's a collaborative website, constantly expanding as new people add pages about their interests, comment on old pages, et cetera.

The original wiki http://c2.com/cgi/wiki might give you an idea of one way a fully-developed wiki might look.

Question about the safety of wikis removed to the WikiPediaFaq. Other discussion removed to BadJokesAndOtherDeletedNonsense.