Wiki is Hawaiian for "quick". Wiki is also a software tool that allows users to freely create and edit hyperlinked Web pages using a web browser. Wiki imlpementations typically use a simple syntax for users to create new pages and crosslinks between pages on the fly. In addition to the main open source version there are also many non-commercial and commercial clones and some "wiki farms" (places where you can set up a wiki without needing your own server) such as SeedWiki.
Wikis are typically used as personal or collaborative content management systems as they allow users to rapidly create, maintain and expand an intricately interlinked network of pages on a particular topic - e.g., a hypertext manual for some or other system. David Mattison (2003) has written a very accessible but comprehensive introduction to wikis and how they can be used. He also reviews a variety of free wiki tools. Another good, short introduction is Wikis described in plain English by Lee Lefever.
Disinfopedia is another, much smaller, Wiki that describes itself as "a collaborative project to produce a directory of public relations firms, think tanks, industry-funded organizations, and industry-friendly experts that work to influence public opinion on behalf of corporations, governments and special interests."
Another nice example is the Moveable Type Knowledgebase wiki - a collaboratively created and maintained collection of how-to information about the Moveable Type software system.
Liftwatch.org is a community portal for people working on the concept of a space elevator and uses TikiWiki, an open source wiki-based content management system.
There are many free open source wiki implementations.
UseModWiki has a reputation for being particularly easy to install and with a good balance between features and ease of use.
Tiki is an open-source wiki-based content management system. The Tiki site has a page devoted to using Tiki for teaching. The TikiWiki homepage is here.
Even though in a typical Wiki anybody can edit (and add) pages, vandalism and "edit wars" are rare, partly because codes of acceptable editing practice tend to arise and partly because it is always possible to restore old versions of a page.
A more important threat to a Wiki is when nobody contributes. They think a core of at least five active contributors is needed to get a Wiki project off the ground.
The downside of wikis
Richard Kulisz (2003) has written a piece on Why Wiki Works Not which sets out a series of problems with using Wikis for collaborative work. His intention is not so much to slam wikis, but to show how and when they don't work well. Two other things to bear in mind when reading the document:
It presupposes a fair amount of background knowledge about both wikis and information theory.
It is itself part of a wiki and therefore a work in progress, with comments and changes constantly being added.
A downside to wikis perhaps not sufficiently highlighted by Kulisz is that they are confusing - with endless links among pages, but not sufficient cues as to a larger conceptual or navigational structure.
Using wikis in learning
James Farmer has a wonderful, simple lesson plan involving getting students to collaboratively construct a mircro encyclopedia (a scaled down version of Wikipedia) on a particular topic.
CourseForum is a commercial wiki system designed specifically for e-learning.
Heather has written a nice short account with the title "My brilliant failure: Wikis in the classroom". The bottom line is "To really use a wiki, the participants need to be in control of the content- you have to give it over fully."