To elaborate on the answer to this question Jack worked for the East India Trading company under Beckett when he was asked to sail an outdated ship. The deal was that the ship would be sold to jack for one shilling after he had delivered the cargo.
This cargo was 100 human slaves.
During the delivery Jack realized he could not complete it and freed the slaves. Because of this Beckett ordered the ship, the Wicked Wench, to be burned while Jack watched. While the ship was burning Jack tried to save it, nearly dying and making a deal with Davy Jones who resurrected the charred remains as The Black Pearl making Jack Captain for 13 years.
After those thirteen years when Jack attempts to free himself from the deal he is given the choice of sacrificing 100 human souls to save his own, the same number of human souls he once freed.
This is revealed in a small yet clever bit of dialog in At World's End between Beckett and Jack.
Becket: "We had a deal, Jack. I contracted you to deliver cargo on my behalf, you chose to liberate it."
Jack: "People aren't cargo, mate."