A verbally shortened version of the verbal "woohoo" became typed due to the limitations early bbs's placed on their users. Over time it grew from woot to w00t because b1ff style typing became "cool"
w00t is an expression of joy or success
such as: "w00t! i won the lottery"
This idea comes from people thinking w00t is a l337ified version of what and w00t stands for what in a question form: "What!? i won the lottery!!"
It originated from "we owned other team"... 'own' generally means to prevail over something
these kind of phrases originated from games such as counter-strike and unreal tournament, nobody is sure of their true origin.
l337 speak itself came from the idea of using numbers and symbols to replace letters so that illegalities could not be found through searches: Hacker became h4xx0r
l337 speak has changed and everybody has their own versions