Forces Acting on an Airplane
Drag is the force that slows the airplane down. The total drag is the result of combining parasite drag and induced drag. Parasite drag has many components, such as form drag, friction, and interference drag. Parasite drag increases as the speed of the aircraft increases. Induced drag is the result of the lift created by the wing and decreases as the speed of the aricraft increases. (Actually, induced drag descreases as the "angle of attack" descreases, which usually occurs when the aircraft speeds up.)
What goes up, must come down.
Lift is the force that must equal gravity to keep a plane at altitude, and must exceed gravity to allow the plane to climb. Lift is created by the shape of the wing moving through air. Thicker air and a faster wing creates more lift.
Thrust is the force created by the aircraft's engine and propeller. Energy delivered via thrust is used to create lift and overcome drag, moving the aircraft forward and up. Some planes use jet engines to produce thrust and have no propeller.