Wook is the 3rd stage of woot
(followed by woop
It's basically another way of saying woop (because everyone says woop!) Althought woop was another way of saying woot (because everyone used to say woot!). And soon it'll likely move on to woog or something simular.
It is a way of expressing happyness or excitement, and ise used by gamers mainly.
<br/>Wook also means a nasty old hippy or something, look below...