A wiki is an online information publishing system that consists of interlinked pages. Wikis are generally editable by the public and have a revisioning system to protect against deletions.

Ward Cunningham, and co-author Bo Leuf, in their book The Wiki Way: Quick Collaboration on the Web described the essence of the Wiki concept as follows:

• A wiki invites all users to edit any page or to create new pages within the wiki Web site, using only a plain-vanilla Web browser without any extra add-ons.
• Wiki promotes meaningful topic associations between different pages by making page link creation almost intuitively easy and showing whether an intended target page exists or not.
• A wiki is not a carefully crafted site for casual visitors. Instead, it seeks to involve the visitor in an ongoing process of creation and collaboration that constantly changes the Web site landscape.

Source:

http://en.wikipedia.org/wiki/Wiki