How An Aero plane Flies ?

Essentially there are 4 aerodynamic forces that act on an airplane in flight; these are lift, drag, thrust and weight (i.e. gravity).

In simple terms, drag is the resistance of air molecules hitting the airplane (thebackward force), thrust is the power of the airplane's engine (theforward force), lift is the upward force and weight is the downward force. So for airplanes to fly and stay airborne, the thrust must be greater than the drag and the lift must be greater than the weight(so as you can see, drag opposes thrust and lift opposes weight).

This is certainly the case when an airplane takes off or climbs. However, when it is in straight and level flight the opposing forces of lift and weight are balanced. During a descent, weight exceeds lift and to slow an airplane drag has to overcome thrust.
The picture below shows how these 4 forces act on an airplane in flight: