A wiki is a web site consisting of pages that can be edited inside the browser. Each change is recorded, providing a full version history of the page and identifying which authors made what contributions to the content. The most popular use for a wiki is to collaboratively create documents using just the web browser.
Installing Wiki software is not for the faint-hearted at the moment, and is unlikely to be commonly adopted in organisations until either one of the large software vendors (Microsoft, IBM, Oracle) produces an application that is easy to install, configure and, more importantly, use, or a smaller vendor grabs enough market share to become noticeable. This is likely to begin during 2006, with some significant early adopters joining Dresdner Kleinwort Wasserstein but is unlikely to spread significantly before 2007. Wikis will become part of an organisation's enterprise content and collaboration strategy, and will need evaluating alongside existing solutions.
The inventor of the wiki is Ward Cunningham. His history of the wiki can be found here. And here is his description of why he chose the word 'wiki'.
Wikipedia provides a good list of available wiki software grouped by programming language.
The O'Reilly Network has a good review of the most commonly used open source wikis.
Microsoft and IBM have both announced wiki intentions. The next version of Microsoft's SharePoint technologies will include wiki-features, as announced here, and are likely to be released before the end of 2006. IBM is likely to do the same with its Lotus Notes and WebSphere software.
One of the most likely contenders in the small vendor category is SocialText. They already have some strong case studies and are looking to be to wikis what Plumtree was to portals.
Instiki is a wiki clone that I use for demonstrating what a wiki can do on my laptop. The installation is simple and the only pre-requisite is that you install Ruby (it runs as a self-contained app, no web server or database required, pretty much OS-independent). Useful if you want to experiment and understand how a wiki works, without committing to a full installation.
Wikipedia is perhaps the most well known (and mature) example of a wiki - it is an online encyclopaedia maintained using a wiki. Anybody with a web browser is free to add, or update, an entry.
The discussion about Wikipedia versus Britannica is beginning to show similarities in emotion to the 'Intelligent Design versus Evolution' debate.
Personally, I don't view Wikipedia in the same way as Britannica. If I look something up in a formal encyclopaedia, I would expect the answer to be 100% accurate - a fact to be used in reference. I use Wikipedia as a quick way to locate a 'good enough' description about something.
A fake entry in Wikipedia kicked off a storm of protest during December 2005.
It all started with an editorial piece in USAToday: A false Wikipedia biography written by the victim of the false biography, John Seigenthaler. Various news sites and bloggers picked up the story. Then the author responsible for the entry was tracked down: Wikipedia prankster confesses (and lost his job in the process)
A couple of good articles appeared, after the storm had passed, highlighting how Wikipedia is changing how we react to incorrect facts and what we should (and shouldn't) do.
Both acknowledged how, for once, the victim of the prank didn't head for the courts (makes a pleasant change) or demand tighter controls (unlike many of the early news articles highlighting the story).
The latest teacup storm over Wikipedia is about whether or not you should be able to edit your own biography (if you happen to have one on Wikipedia).