It can be supplied by propellers or engines, and must be great enough to provide sufficient airflow over the wings. Propellers are basically twisted wings, and provide in a forward direction what wings provide in an upward direction. The drag is basically the force pulling the aircraft back, opposite the direction of thrust. There are three different types of drag which combine to form the total drag. These are friction drag, form drag, and induced drag.
Form drag is just the air's resistance to the plane passing through it.
The more aerodynamically shaped the plane is, the less form drag is created. The third type, induced drag, is caused by the higher pressure air, created by the lift, trying to rush into the area occupied by the lower pressure air. The secret to how a wing creates lift is in its shape. The oncoming air has a greater distance to travel over the top of the wing and, thus, travels faster above the wing than it does below. The idea that increasing the speed of airflow decreases the pressure is known as Bernoulli's Principle.