I've had a number of people talk to me lately about wikis, and from these conversations it has become clear to me that at least a few are unclear on what they are about. So, I'll take a stab at defining them.
First and foremost, wikis are about authoring a set of static HTML pages. That's it. If you're trying to do something with a wiki that would be hard to do well with a set of free form HTML pages written using Front Page, then you probably won't do it well with a wiki. A great example of this is discussions: wikis pretty much suck at threaded, back-and-forth conversations, because free form, static web pages pretty much suck at this. Note that more constrained or more dynamic web applications might still do a decent job (think message boards), but a big ol' pile o' HTML is a long way from being a discussion board.
- A simple authoring language.
The collaboration one is the one you generally hear about most. The idea that lots of people can walk up to your website and author their very own page, or even edit one that you wrote is totally crazy-sounding, but turns out to be immensely powerful.in a lot of situations. Of course, because we can't trust everyone not to deface the shared page, most (not all) wiki engines offer version control to make it easy to undo what others have done, but I don't think version control is really central to the Zen of wiki.
However, I happen to think that the other major facet of wikis is every bit as important as the first: the fact that they introduce a simple authoring language. For example, if you look at the language that FlexWiki offers, it's simple do make something bold - you just *surround it with asterisks*. The wiki engine then interprets this into HTML that makes the enclosed text bold when the page is viewed. While this isn't much simpler than surrounding the same text with the equivalent <b> tags, it turns out that this syntax (and the other syntactic elements a good wiki language offers) are just _enough_ simpler to lower the bar on contributions to the point where people are actually willing to spew their brains onto a page.
It really says a lot about human laziness that harnessing its power can do so much. :)