A web based repository of knowledge built using a wiki technology in multiple languages through a large collaborative effort about topics chosen by the participants.
For its own description of itself go to http://en.wikipedia.org/wiki/Wikipedia. This description is fairly in depth, but it is unlikely to include information which lots of people disagree with. However, this wiki project reached critical mass and is a self motivating community of knowledge editors who must only be agreed with to succeed. It is very useful for looking up information about practically anything in any language.
Wikipedia is notably the largest competitor in some sense of the word to Knol.