OnLine Editing

Wikis, GoogleDocs, and now Buzzword Documents all provide ways to share the editing of an online document (as do a number of other options).
What are the main differences? The latter two are company-hosted and provide substantially complete word processor features for preparation of a print document with options to save in several common formats, whereas the wiki is definitely an html document (with perhaps more rudimentary formatting options but hyperlinks encouraged rather than just being possible as in Word), and provides perhaps more detailed capability for change tracking and reversion. Also, the wiki is more generically defined and can be self-hosted using any of several available implementations. Of course, as feature sets expand they will also overlap more, so the distinctions may become even more subtle over time.