Wikis let tech-savvy people all over the world collaborate on a single web page, a single document. That's a pretty powerful idea, really. The obvious thing to do once an idea like wikis appears is to attempt to cram all of humanity's knowledge into it. Well that's just crazy enough to work.
Wikipedia is the gold standard of wikis. It's a collaborative encyclopedia with lots and lots of articles. As the gold standard, it influences people's thinking about wikis in ways that might not be quite true.
If Wikipedia were the only wiki you'd ever heard of, you might think that wikis are used to create encyclopedias. If that were true, you'd really only need one of them.
If Wikipedia were the only wiki you'd ever heard of, you might be under the false impression that all wikis everywhere are editable by anyone. So if you create a wiki page, it's no longer yours. Other people can scribble on it. Well, some kinds of documents perhaps just aren't suitable for being edited by anyone, and the good news is that you can build a wiki for you and your classmates, or you and your coworkers, and you can decide who gets to edit what.
I want to think about a few distinct uses for wikis in different contexts. The most common use is of course to create a knowledge repository; a collection of information written by people who know what they're talking about. That's what Wikipedia is, mostly. That's what the Haskell Prime wiki (which I run) is. And the excellent wiki about the game of Go, Sensei's Library, is also a knowledge repository.
But there are other uses for wikis that are pretty cool. I often have to work on proposals with my coworkers. One (horrible) way to do this is to write the first draft in MS Word, email it to everyone, who might edit it and email it back. Of course, only one person can actually edit it at a time, otherwise I have to figure out what everyone did and merge their changes by hand. That's what we, in the computer industry, call teh suck.
A better way to do this is to use a version control system, which lets multiple people work on a document at one time. Of course, if you're editing evil document formats like Word, still only one person can edit it at a time, but at least you cut out email, so things are a bit better organized.
But wikis are actually a really great way to work on a document with another person. Two or more people can each collaborate on the document using the wiki software over the web. What becomes of the document at that point depends on its ultimate use. Maybe you have to convert it to MS Word or PDF at the end. That's a bit of a bummer.
So there are two uses of wikis that I've mentioned: building a knowledge repository, and collaborative writing.
Google started a service for collaborating on documents called Google Docs & Spreadsheets, which is probably useful for collaborative writing, but not knowledge repositories. They also acquired a wiki startup a few days ago, so they are clearly trying to get all over this space.
There's another very interesting use of wikis that I'll call situation awareness. You can get a really vivid picture of this use of wikis if you take a look at the wikipedia article on the July 2005 London bombings. By the way, I didn't discover this excellent example of situation awareness using wikis. I saw it in the slides for a talk, but I don't know the original source at this point.
If you go to that link, you'll see an excellent and up-to-date encyclopedia article about the London bombings. It's not a news article. Go to Google News and try to find a news article from a major news source which is this detailed, up-to-date, and well organized. Most articles you'll find are a snapshot, taken at a particular point in time (when the article was published) that most likely contains an account of the latest developments, and usually brief background in case you missed previous stories.
The wikipedia article isn't a snapshot, it's an article that explains things in a logical order, which isn't necessarily chronological.
Each wikipedia article contains a link to its own history. You can look back over time and see the article evolve. If you look at the early edit history of that London bombing article, you'll notice something fascinating. The article was created shortly after the bombing, with an account of the information that was known at that time. Within an hour, many people had edited the article, adding information they know about. If you were refreshing that wikipedia entry over & over (or if you were subscribed to it using RSS), you'd see not an account of just the latest developments, but an evolving, logical understanding of the attacks. The article started evolving right away, and it continues to evolve to this moment.
That's what I'm calling situation awareness. Don't get me wrong, the mainstream news media is quite good at situation awareness. They do it a bit differently, and their articles or TV stories don't become integrated into a knowledge repository like wikipedia.
I've enumerated three somewhat overlapping uses for wikis that I think are pretty interesting to examine: