An excellent description of what a wiki is, and what it's used for, all in non-technical terms. Todd asked about wikis in the forum here.
Ultimately, a wiki is a specific type of website. A wiki is special because it allows a group of people to build, edit and modify a website with no programming or HTML whatsoever. Because it doesn’t require technical expertise, all users of the wiki have equal ability to maintain and edit the site. Wikis are easy to learn and use, which makes them accessible to everyone. Common Craft: Wikis Described in Plain English
I also like this section:
Wikis are different from normal web sites:
Normal web sites are usually developed offline and then presented to users as a finished product. By comparison, wikis are first presented to users as a blank slate- an empty page. A new wiki is an empty wiki, no pages, no links. Instead of a team of designers developing a web site in private, a wiki is developed in public by the users of the wiki over the life of the wiki.