Air charter is the business of renting an entire aircraft (i.e., chartering) as opposed to individual aircraft seats (i.e., purchasing a ticket through a traditional airline). While the airlines specialize in selling transportation by the seat, air charter companies focus on individual private aircraft and itineraries, urgent or time-sensitive cargo, air ambulance service, and other forms of ad hoc air transportation. Some air charter companies offer a large variety of aircraft, such as helicopters and business jets. Charter jet categories include turbo props, light jets, mid-size jets, super mid-size jets, heavy jets, and airliners.