A wiki is an editable website, the word wiki comes from wikiwiki the Hawaiian word for quick. A wiki is a website that is quick and easy to edit. The most famous and popular wiki is Wikipedia.
Years ago I saw the founder of Wikipedia, Jimmy Wales speak, and he said he started wikipedia to give everyone access to the sum of all human knowledge. He wanted there to be an encyclopaedia that has information about anything and is available to anyone.
Not only can wikis be used to produce the worlds biggest encyclopaedia, wikis can be used by anyone to collaborate on a project, an idea or documents and best of all they are free.
And while they were at it the lovely ForwardIT wrote an article about how to create and edit a Wikipedia article. So if there is something that you think needs to be online particularly on Wikipedia, you can add it to the sum of all human knowledge.
There are many wikis available online on a huge range of topics but my personal favourite is the Muppet Wiki (enjoy).