What is a Wiki?
Invented by Ward Cunningham, wikis are a read/write web technology that allow for easy, fast, and collaborative websites to be built without the need for special software or a lot of training. A wiki is a web-based tool that trades simplicity in design for sophisticated multi-user publishing capability--all from a common web browser.
As such, a wiki can be used in three basic ways.
Simple Web Publishing
First, a wiki can serve as an easy web-publishing tool that is managed by a single individual. Whereas a blog can serve a simlar function, a blog has an inherent chronological structure which is limiting. A wiki, on the other hand, has the capacity to allow for the organization of data in either a hierarchical or hyperlink fashion, according to the designs of the publisher. With no expense for a web publishing program, and with the independence of being able to work from any computer with a connection to the Internet and a web browser, a wiki is an incredibly effective tool for writing to the web.
Two Can Tango
Second, a wiki can be "partially collaborative." Multiple individuals, again only requiring access to a web browser, can participate together in the building of information in one website. In a "partially collaborative" wiki, while they are publishing together to a single website, their content does not overlap and may be delegated or assigned.
Third (and arguably the "secret sauce" to wonderful world of wikis) a wiki can be "fully collaborative." In this method of using a wiki, multiple individuals work together and often work on the same content. While it might seem that allowing many people the ability to work on, modify, or overwrite each others work would result in chaos, it typically results in the participants choosing to write in a thoughtful, non-partisan fashion so that others will feel comfortable with the content and minimizing the need for a tug-of-war. Most wiki software allows for a mirrored "discussion" page for each page of content, where contributors can actually talk over the content of the page and their feelings about how it should be presented. A good example of a "fully collaborative" wiki that many people are familiar with is Wikipedia. Wikipedia allows anyone to edit any page that they want to, with pretty amazing results.
Why It Works
There are two safety features (I like to call them "anchors") that allow the deep and free level of collaboration in wikis. The first is the ability to see every change that has taken place on every page of a wiki--either in the history tab of an individual page, or in an overall view of the wiki's changes. Thus, nothing is lost forever, and changes can be undone. The second feature is the ability to "watch" particular content pages, which means that the wiki software alerts the user to any changes that have taken place on a page or pages that he or she cares about. This helps to explain to the uninitiated how it is that an error in a site like Wikipedia can be corrected in a matter of a few minutes.
Wikis are an amazing technology. You can try them out (for free, of course), at www.wikispaces.com or www.pbwiki.com. My site for SupportBlogging! is a Wikispaces wiki--if you want to see what that is like, you can go to http://www.supportblogging.com. An example of a PBWiki site is the wiki for coordinating our efforts at the NECC 2006 show in San Diego: http://necc2006.pbwiki.com.