In layman's terms a wiki is a website that allows users to add content (a bit like a net forum), but where it differs is that it allows the users to also edit any content that is already on the site. In theory it means that everyone can be an editor of a wiki website.
An example of this is wikipedia which is an online encyclopedia where users can refine, make corrections, and generally make the entries more accurate.
If you want to know more about wiki then why not go to the wiki enclyclopedia en.wikipedia.org/wiki/Wiki and read about it.
Apparently wiki is going to be the next big thing, much in the way blogs are last years big thing. remember you read it here first (maybe?)