What is terminal velocity and what causes it?

Initially, when you drop an object the only force acting on it is gravity. This accelerates the object towards the ground. As the objects speed increases so does the force of air resitance acting on the object. Eventually the force of air resitance acting upwards becomes equal to the force of gravity acting downwards. At this point, there is zero net force acting on the object and so it moves at a constant speed. It is this constant speed we call terminal velocity.