Trading cards usually come in packages and often contain pictures and stats of athletes on sports teams. For example hockey, baseball, football etc... When you buy a package of say, 5 cards you don't know which ones you'll get until you open it because they are random. For this reason people often trade cards in order to acquire a full set i.e. every member of every team!

For example let's say I have 2 @papa-pepper cards and someone else has 2 @meeterboom's. We could trade. If a card is very rare we might have to trade 3 for one and so on!

It's huge in North America.

This is a huge producer of trading cards. http://www.upperdeck.com