Pay Per Sale (PPS) is when you only have to pay for an advert when someone makes a sale because of that ad.
That means that you could get someone to display your ad and have 1,000 people see it. 100 people clicked on the link. 10 people signed up to your monthly new paper. 1 person brought a product/service. You would only have to pay for that one person because they are the only one that actually completed a sale.