A passport is a document that specifically identifies you as an American citizen. It allows you to travel in and out of foreign countries and to have access to their governments' lawful protection.
Your passport must contain a photo of yourself. This is a security measure that prevents someone from stealing your passport and using it under false pretenses.
A Passport Card only lets you enter Canada, Mexico, Bermuda and the Caribbean by land or sea, whereas a Passport Book is valid for other countries and also for air travel. Both contain the same identifying information, such as your name, place and date of birth, and gender.
Your passport contains an expiration date. You must renew it before that date or it will be considered invalid. If you were older than 16 when you obtained your passport, it will remain valid for 10 years.
Your passport should contain your signature attesting to the accuracy of the listed information, and an emergency contact. If the information isn't correct, do not sign it, and return it to your state office.