Every particle in the universe is subject to force. Each particle attempts to reach its stable position, by balancing out various forces acting on it.
In case of flight, there are 4 such forces, as follows:
Weight is the force of gravity. It acts in a downward direction—toward the center of the Earth.
Lift is the force that acts at a right angle to the direction of motion through the air. Lift is created by differences in air pressure above and below the wing of airplanes.
Thrust is the force that moves the flying machine in the direction of motion. Engines produce thrust with the help of propeller, jet engine or rocket.
Drag is the force that acts opposite to the direction of motion. Drag tends to slow down the flying machine. It is caused by friction and difference in air pressure.