The term 'Wiki' is the Hawaiian-language term for 'quick'. Wikis are a form of collaboratively-editable websites, the most famous of which is Wikipedia. While many CMS tools also allow for collaborative editing of content, Wikis excel at unrestricted and free-form creation. Wikis generally use a simplified markup format, known as Wiki-text and allow for links to pages and topics that do not yet exist. Rather than providing a detailed authorization system to limit who can modify what content, Wikis will instead allow anyone to make changes, but make it easy to roll-back vandalism. This allows wikis to more easily draw a large number of contributers than a more restrictive system, at the expense of control over user-actions.
Other Wiki Software
On the Wiki Matrix you can compare the features of the vast majority of common wiki platforms.