A wiki is a database of pages which visitors can edit live via the internet. They typically allow all users to edit any page, with full freedom to edit, change and delete the work of previous authors.
Another way to understand a wiki is as a document that is collectively created and maintained.
This links to a short (3 minute) video from ZDnet, entitled "What is a Wiki?":
Here is an interesting example of a wiki, a site that is a dynamically written novel:
Here is another wiki, which serves as an online university:
This site lists dozens of other wiki examples: