Drag and Lift
The relative wind acting on the airplane produces a certain amount of force which is called (unsurprisingly) the total aerodynamic force. This force can be resolved into components, called lift and drag.
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Lift is the component of aerodynamic force perpendicular to the relative wind. |
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Drag is the component of aerodynamic force parallel to the relative wind. |
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Weight is the force directed downward from the center of mass of the airplane towards the center of the earth. It is proportional to the mass of the airplane times the strength of the gravitational field. |
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Thrust is the force produced by the engine. It is directed forward along the axis of the engine (which is usually more or less parallel to the long axis of the airplane). |
These are the official definitions.
References: http://en.wikipedia.org/wiki/Form_drag