I was asked in a meeting today what a Wiki is, and I gave an example of Wikipedia to illustrate what one is. Here's a brief description from Nature Magazine that I found through CorporateBlogging.info
One of the extraordinary stories of the Internet age is that of Wikipedia, a free online encyclopaedia that anyone can edit.
Wikipedia has added 3.7 million articles in 200 languages since it was founded in 2001. The English version has more than 45,000 registered users, and added about 1,500 new articles every day of October 2005. Wikipedia has become the 37th most visited website, according to Alexa, a web ranking service.