FOUR MAIN FORCES DURING FLIGHT

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 1. Weight
Weight is a force caused by gravity. Gravity is a special kind of acceleration; it describes the attractive force between the earth and objects on and around it. This attractive force 'pulls' objects towards the Earth and in turn, 'pulls' the Earth towards objects. For an object to go up, the force pushing it up must exceed the force of weight an object has.  http://howthingsfly.si.edu/forces-flight
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 2. Lift 
Lift is the force acting upon an object that counteracts weight. Lift is created by how the air stream around an object interacts with the shape and angle the wings and body of the object. 

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3. Drag 
Drag is a quantitative summary of the friction an object has with the surfaces and objects around it. Drag is the force that Thrust must overcome in order for the object to move forward. During flight, changes in air pressure may also affect the magnitude of the Drag force. 

4. Thrust 
Thrust is the force that we, as operators of an object, can best control. The thrust force is typically created by propellers or engines - something that forces air movement. The force of the thrust created by these mechanisms must overcome the drag force if the object intends to move forward.