What is the diffference between a passport book and a passport card?
As the other two answers have hinted at, unless you live near the US border or you're traveling by sea vessel, the passport card is rubbish at its stated purpose. However, I still have a passport card anyway, I use it as my primary form of ID, and I even travel with it overseas in addition to my passport. Here's why: 1) It's perfectly valid as identification in the US, and it doesn't have your address on it the way driver's licenses do. I've used it to get into bars, cash checks, and get through airport security. And actually, you can use the passport card to get on a domestic flight. You might have to, soon, if you don't have a REAL ID compliant driver's license. 2) It's a good backup in case you lose your passport while traveling. 3) It works pretty well as ID in Europe as well. They're more used to national identity cards and they see that it has a photo and "U.S. Department of State" on the front, and a machine readable zone on the back. You can't actually use it to enter Europe, but besides that they seem to know what it is and that it's perfectly valid. 4) It's unknown enough that it makes a fun conversation starter.