What is a Wiki?
Wikipedia calls it a collaborative website that can be edited by anyone with access to it. Wikis encourage the democratic use of the Web and promote content composition by nontechnical users.
The first wikis appeared in the mid 90's as scientists and engineers used them as knowledge databases. Adapted more widely in the last few years, wikis are used for a variety of collaborative activities: as a repository for meeting notes, as a way of building an agenda, and for classroom and faculty group projects.
Here are two entertaining videos on wikis. Check out Wikis in Plain English, and Wiki Man for an introduction.
The document, Seven things you should know about wikis, outlines implications for teaching and learning, as well as some of the downsides of wikis.