"A wiki, a free form, unstructured space, is like a room. A room can be used for anything. For meetings, for sleeping, for swimming (provided it has a pool), but a room is just a space. And to the people using the room, that space becomes a place. A place to situation themselves, a place to call home, a place to organize themselves, or a place to orient from."
Sunir Shah's explanation of the Wiki-phenomenon by analogy to a room is possibly the most precise write-up on this topic yet. It's so good, it would have to be quoted completely - so better go and read it now!