A wiki is a webpage which allows its users and viewers to easily edit and create any of its pages. It can be freely edited by the sites users, without permission, usually with the purpose to build a community-maintainable webpage or encyclopedia.
Wikis usually have a special, easy to learn markup. For example, MediaWiki, the software this wiki runs on, allows its users to create a link to another wiki page by surrounding the linked word with square brackets (such as [[Foobar Rhinoceros]]).
For more information, view Wikipedia's article on wikis.