He was killed in a drive-by shooting. His grandfather found him, raised him from the dead- Jack being resurrected so shortly after his death that he never showed any sign of being dead apart from the bullet holes under his shirt-, and presumably taught him how to raise his friends. He attempted to 'recruit' Xander Harris as the group's driver as he led a gang of zombies whose intent was to blow up Sunnydale High, but Xander fled from the group when they tried to kill him. After Xander disarmed the bomb, he fled from Jack through the school, only for Jack to be eaten by Oz while Oz was in wolf form.
As a zombie, Jack O'Toole had no need of food, drink, or oxygen, and was immune to most physical damage, although he did not show any sign of being able to heal as he retained the bullet-wounds that had killed him.
He was also skilled in magic, as he was able to raise other zombies from the dead.