1) An Expression of joy generally used by computer gamers and hackers.
2) An exclamation of victory or triumph over an obstacle such as winning a large prize at a LAN party.
3) A part of the 1337 language.
History: The current-day use of the word w00t stems from hackers in the early to mid 80's. While communicating with each other groups of hackers such as Razor1911 would need lingo which nobody else would be able to understand to express milestones in their hacking. One such milestone was gaining root access, but the term rooted or "gained root access" was easily understood so the term was changed to w00t to help disguise. Because of the difficulty of "rooting" many times the term w00t would be much in a celebratory tone. It later evolved to simply be a celebratory remark rather than a hacking milestone.
#2 after winning the Radeon X1900 XTX, Billy said "I am teh 1337... w00t!!"
#3 well I keylogged and doom9'd his box and... yep, w00t!!!