To many, planes fly because they go fast and they have wings. When I was younger, that is how simple I thought it was.
Well, there is a little more to it than that. There are four forces of flight: lift, drag, weight, and thrust, which correspond to upward, backward, downward, and forward forces, respectively.
Thrust is what moves the aircraft forward through the air, it overcomes the drag and the weight of the plane. The thrust for a normal plane comes from an engine/propulsion systems such as a propellor, turbine or a rocket.
Weight is simply the force on the airplane caused by gravity.
Drag is the force that opposes the plane's motion through the air and is generated on every part of the airplane. There is even drag that is caused by the generation of lift called induced drag.
Lift is a very complicated force. It is the force that is the opposite of the plane's weight and it holds the plane in the air. Some lift is generated all over the entire plane, but a majority of it is generated on the airplane's wings. This is what we will discuss a little further, because lift is extremely interesting:
How is lift generated? Lift occurs when there is a flow of a fluid turned by a solid. A fluid can actually be catorgorized as either a liquid or gas, so when lift occurs, the plane and its wings turn the air. The wings are designed so that a low pressure area is developed with air that moves very quickly along the top of the solid, and a high pressure area is developed with air that moves more slowly along the bottom of the solid. The result is an upward movement as the high pressure pushes the plane into low pressure with an equal, opposite force.
Needless to say, planes have incredibly well designed wings that create lift, which is the vital part to why planes fly, and is very awesome to see the physics behind!