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...
Wooks tend to travel in packs, they smell strongly of patchouli and are in constant search for free drugs. One of the defining characteristics is an excessive amount of unkempt hair, usually in dreadlocks.
It is important to make the distinction between a hippie and a wook. Hippies can generally be viewed as positive, optimistic members of society with an idealistic goal for the betterment of society. Wooks are everything that you've been warned about in regard to hippies wrapped into a neat little package.