A wiki is a web page that lets multiple people collaborate on its content. One of the best-known wiki sites is Wikipedia, a collection of encyclopedia articles added and maintained in a wiki. This EcuLab site is actually a basic wiki.

There are lots of wiki sites (or wiki farms) available, with varying features and costs. A comparison of features and pricing can be found in this Wikipedia article.  If you want to compare non-hosted wiki software alongside wiki farms, take a look at http://www.wikimatrix.org/.