A wiki is a document that is collectively created and maintained. Anyone can edit a wiki. This feature seems to evoke a sense of responsibility and seriousness among most Internet surfers. It’s a great way to involve users in a site, allowing the users to build something useful for everyone.
Surprisingly, there were few problems with people abusing wiki. Because modifications are both easy and expected, there’s no challenge to “breaking” and defacing a page. And that same ease of modification allows anyone to rollback a modification if someone does deface a page.
Also, Check out the definition at WikiPedia to find out more.