Ben Lanterman hit the nail on the head. When I was in my aviation classes at Purdue, this was one of the quiz questions we got all the time. The answer that people are quick to jump on is that "the airspeed is to low and the wing is not producing enough lift to keep the aircraft in the air. While this is true, there is more to it than that as Ben states. A stall can happen at any speed, high or low. A stall occurs when the angle of attack causes a disruption of airflow over the top of the wing resulting in the loss of lift. Three types of stalls: Departure Stall(Power on Stall), Arrival Stall(power off stall), and Accelerated Stalls.