An online encyclopaedia.
The key differenece between wikipedaia and other online references such as Encarta and Brittanica is that all the content has been supplied by the public and is also edited by the public.
The fact that anyone could change the content of a wiki page drew criticism that it was inaccurate. So to address this, some of the more commonly vandalised pages can now only be edited by approved editors.
The software that drives wikipedia is free and open source. There is nothing to stop you putting up your own 'wiki' for some special interest group, but you do need to be prepared to keep an eye on the pages because of vandalism.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Wikipedia