What is the passport card?? Well here is the definition from http://www.travel.state.gov ..
The passport card is a wallet-size card that can only be used for land and sea travel between the United States and Canada, Mexico, the Caribbean, and Bermuda. The card will provide a less expensive, smaller, and more convenient alternative to the passport book for those who travel frequently to these destinations by land or by sea.
That's right its less expensive!! If you only travel to these countries why pay for the regular passport?? The cost of the passport card is only $45 for adults and $35 for children. I find so many people are buying the regular passport. This is a great alternative and really can save yourself money for those shore excursions!!