Wikis are websites that anyone can edit. The most famous wiki is Wikipedia.
Wikis are a hallmark technology of Web 2.0, in that they are based on the collective knowledge and contributions of their users. Thorough repositories of knowledge can easily be created in wikis, even if these individual contributions are relatively small. Wikis also serve as open forums for debate regarding whatever knowledge users are dealing with, and features such as discussion and history pages allow for a meta-examination of how individual wiki articles have changed over time.