LIFT

    THRUST     DRAG

                                             WEIGHT

For a plane to take flight the LIFT and THRUST must overcome the WEIGHT and DRAG to force the plane forward and upwards.

Likewise for a plane to fly straight and level Thrust = Drag and Lift = Weight

                                            LIFT

  AIRFLOW

As air hits the wing, some of it goes on the top while the rest goes on the bottom. The air on the top is accelerated and lose air pressure, this causes the wing to move upwards due to greater air pressure underneath the wing.

References

http://www.aeromuseum.org/Education/Lessons/HowPlaneFly/HowPlaneFly.html

http://library.thinkquest.org/03oct/00181/airplane-t.html

http://www.atac.ca/FunFacts/How_Flies.htm