Any plastic card (like a credit card) with an embedded integrated circuit for storing information.
Smart cards are being incorporated into soldier's dog-tags and used to store hospital patients' medical records. This way they are always instantly accessible.
Other uses are as a form of token in banking systems. You could store electronic money on the card or less valuable tokens such as those given away by petrol companies which you collect to exchange for free gifts at a later date. The idea being that one smart card is easier to carry around than a multitude of paper tokens.