What's the difference between a passport card and a passport book?

Question

http://travel.state.gov/passport/get/fees/fees_837.html

I'm confused...

Answer 1

You can only use a passport card to travel by land within North America or by sea in the Caribbean region.  A passport book can be used to travel anywhere in the world by any means...land , sea, or air.

Answer 2

A passport book can be used anywhere,The passport card facilitates entry and expedites document processing at U.S. land and sea ports-of-entry when arriving from Canada, Mexico, the Caribbean and Bermuda.  The card may not be used to travel by air.  Otherwise, it carries the rights and privileges of the U.S. passport book and is adjudicated to the exact same standards.