In addition to a regular passport, (the Passport book), U.S. citizens have the option of using a smaller, lighter passport card but only if they are traveling by land or sea to certain countries. A passport card is cheaper to get but is only valid for travel to Canada, Mexico and the Caribbean islands. The passport book is valid for travel anywhere and is required for international air travel.
A passport book measures 5"x3.5" when closed. It contains several pages like the passport details page which provides the holder's name,date of birth, passport expiry date etc as well as other blank pages for visa and immigration stamping.
A passport card is a portable pocket sized card (similar to a credit card) that fits in your wallet.
A passport book is valid for any international travel by air, sea or land. It is valid for 10 years for adults and 5 years for minors under 16.
A passport card is valid for travel to and from Canada, Mexico, the Caribbean and Bermuda at land border crossings and sea ports-of-entry. It is not valid for air travel. It is valid for 10 years for adults and 5 years for minors under 16.
A first passport book costs $135 for adults and $105 for minors under the age of 16. It costs $110 to renew.
A first passport card costs $55 for adults and $40 for minors under the age of 16. It costs $30 to renew.
The cost when applying for both is $165 for adults and $120 for minors.
edit Flying Without a Passport (Book)
Q: I am taking a cruise to Mexico and I intend to travel with my passport card because I'm traveling by sea. If I fall sick in Mexico, can I fly back to the U.S.?
A: No. You need a passport book for air travel. The passport card is only valid for entry into the United States at land border crossings or sea ports of entry.