|There is a Wikipedia article about trading cards. They are small cardboard cards with images of persons on it. Often those are well known sports stars. The idea is to collect them all. The business modell behind commerical trading cards is to make some of them scarce so that you have to buy loads of those less scarce ending up with lots of duplicates and still lacking the rare ones. You will try to trade them with fellow collectors, offering much more of the less scarce cards for a scarce one. Often they are used as incentive to buy some unrelated product, e.g. breakfast cereals. You know there is a trading card in the box, but you don't know which. In this way you end up buying 10 times as much breakfast cereal boxes than you can eat.