when early hackers broke into a system - they reached the "root".. so the would procliam or yell "root!" and then that turned into "woot!" because it sounds like root. and now with all the 1337 h@xor type, its spelled "w00t"
its just a general word used by computer geeks now to show excitement and happiness for doing something good.
in other words, its retarded. its a nef.