Woot originated as a hacker term for root (or administrative) access to a computer. However, with the term as coincides with the gamer term, "w00t".
"w00t" was originally an trunicated expression common among players of Dungeons and Dragons tabletop role-playing game for "Wow, loot!" Thus the term passed into the net-culture where it thrived in video game communities and lost its original meaning and is used simply as a term of excitement.
"I defeated the dark sorcerer! Woot!"
"woot! i r teh flagmastar!" (Think Tribes)
"Woot, I pwnzed this dude's boxen!'