A wiki is website just like any other website which allows guest and registered users to add, modify, edit content. Anyone can modify content. It follows online collaboration model and good for building wiki. Sometime wiki also refers to the collaborative software such as wikimedia.
According to wikipedia:
The first wiki (WikiWikiWeb) was developed by Ward Cunningham in the mid-1990. The word wiki is a Hawaiian word meaning “quick”. Wiki systems are therefore designed so that their content can be made available in a quick and uncomplicated manner.