Paul,

A slot card is similar to a credit card in appearance (only!). At nearly every casino you receive one by going to the promotions booth and they will issue one to you. If your playing slots, you normally insert it into the machine to track your play. At table games, you play is normally tracked by presenting your card to the pit personnel, who will swipe it through a reader and manually track your play. All this leads up to the casino having some basis for rewarding you play with comps.

Many folks collect them as each casino issues its own and the artwork can be pretty nice.

Hope this helps, Bob