"[Wikis] are also cool because they are the antithesis of traditional enterprise software with its top-down design the imposes process, ontology and structure upon users. By giving users the power to create, link and form groups it serves the domain of business practice, the unstructured collaboration that leverages informal networks. A wiki can serve group activities quickly, so a project can begin with conversation and prototyping instead of waiting for a tool to be created or implemented."
-- Ross Mayfield
Whereas much of the interview is (understandably ;-) a SocialText semi-commercial, the first few paragraphs summarize the basic concepts behind Wikis very well.