AirTran notes that TSA regulations allow each traveler to carry one bag, which will be stored under the seat, plus one carry-on. The bag can be a standard-sized duffel bag or backpack, while the additional item should be smaller.
Acceptable carry-on items include laptop bags and purses, and similarly sized items. Airlines typically let travelers check one bag for the price of their travel ticket but charge an additional fee for extra bags. Items that don't count toward the limit include umbrellas, mobility aids, child restraint seats, wearable clothing such as a coat, and food or snacks meant for consumption on the plane.