All things that fly using wings need air. Moving air has a force that will lift kites, airplanes, and balloons up and down. An aircraft experiences four basic forces, they are
Lift: The upward force acting on the aircraft which helps the airplane to fly.
Drag: The backward force caused by the resistance of air flow.
Thrust: The forward force produced by the engines of the aircraft
Weight: The body and cargo weight of the aircraft acting in a downward direction.
An aircraft flies when the lift force exceeds the weight of the aircraft. For this to happen the aircraft must move at high velocities. The engines move the aircraft forwards by providing thrust and the wings make it fly by providing lift.
Lift is created when a pressure difference occurs between the upper and lower surface of the wing. In other words, more air molecules should crash the underside of the aircraft than the upper side. To achieve the pressure difference the wings are shaped in the form of aerofoil as shown below (side view of the cross section of the wing).
The wing is slightly inclined with the plane of the flight path. This allows the lower surface to face more air molecules than the upper surface. When stationary, the wing experiences an equal amount of pressure on the top and bottom surfaces.
The engine propels the aircraft to high speeds. As the speed increases, the force acting on the bottom of the wing becomes larger than the force acting on the upper surface due to the pressure difference.
The air at the upper surface travels faster over the airfoil compared to the air at the bottom surface causing a difference in pressure. As the air flows over the upper surface which is curved, the air tries to move in a straight line, but the curved shape of the airfoil pulls it around and back down.
For this reason, the air is stretched out into a larger volume and the same number of air molecules forced to occupy more space when compared with the air molecules at the bottom surface and this is what lowers the pressure. This condition lifts the aircraft up from the ground.
As the F(lift) is more than F(gravity), where "F" represents the force, the aircraft moves upward. The wings are installed with various control surfaces like flaps, slats, ailerons etc. which change the shape of the aerofoil, thus aiding the change of lift forces. After reaching the required height, the wing shapes are adjusted and speed of the aircraft is maintained such that the lift and weight of the aircraft are balanced which enables in cruising at constant altitudes.