Air resistance opposes an object falling through the air. It is the frictional force of the air, acting on the body until the resultant force is zero, i.e when the body has no acceleration. When air resistance equals the weight of the object acting downwards the object falls with a constant speed called the terminal velocity. The value of the terminal velocity depends on the size, shape and weight of the object.
A small dense object has a high terminal velocity and falls a considerable distance with a constant acceleration of about 9.8ms^-2 before air resistance equals its weight, whereas a large light object has a low terminal velocity and very quickly air resistance equals its weight. The changes in terminal velocity is due to the surface area that the air can act on and not the mass of the object which used to be believed. Galileo was one of the first to suggest the air resitance affected a falling object and not its mass. This was proven in 1970 when the team of Apollo 15 on the moon dropped a feather and a hammer from the same height and they both hit the ground at the same time. Without air resistance the accleration was the same. Without air resistance on the earth all objects would accelerate at 9.8 ms^-2 and hit the ground at the same time if dropped from the same position.