A wiki is a website where you can create and edit a collection of interlinked web pages. Wiki is a Haiwaiin word and means fast or quick.
The largest example of a wiki in the world is of course Wikipedia which is a free online encyclopedia that is written collaboratively by the people who use it. Anyone can edit almost every page and as all changes are recorded, updates can be verified and revered if needs be. To find more about updating or writing a new article look here.
The Oxford Internet Institute at the University of Oxford has created the visualisation below of the distribution of Wikipedia articles by country.