When flying domestically, meaning within the United States, all that is necessary is a state-issued identification card, according to the TSA, the Transportation Security Administration. The nifty thing about a passport is that it be substituted for your birth certificate and social security card in most cases, and it can be used for more than just travel.
Only a passport is required for international travel, though it is always wise to keep your state-issued ID card accessible in case of emergency. If you are detained in a foreign country, those forms of identification can ease the process of getting home. Of course, if a passport is lost or stolen, having a state-issued card will ease the process of getting a replacement passport as well.
Per the TSA, other acceptable forms of identification are cards that show name, birth date, expiration date and gender, and include government-issued security features such as holograms and watermarks.
