A Wiki as some kind of collaborative writing, because each and everyone of us (to use Morpheus words) can change the content. Isn't this worth writing for? Isn't this worth hosting for? Ok i fade away...
There are much better descriptions of what a wiki is out there. For example you could visit the father of the wikis or the famous wikipedia. Other popular examples are meatball and infoAnarchy.
The inventor of wiki is Ward Cunningham