Center of Gravity, and Center of Lift

Center of Gravity (CG)

Also called Center of Mass.

If we could concentrate all the airplane mass into a single ball, with the exact same weight, where it will be located to represent the same effect as the whole airplane is the CG.

In other words, if we look on the four forces that work on an airplane during flight, the CG is the point the Gravity arrow pull it down. 

Center of Lift (CL)

All the partial lift powers that are generated from the wings can be replaced with one collective force, generated from the Center of Lift (CL).

Stability, CG and CL

The relations between the CL and the CG locations determine the stability of the airplane.

See this example of a stable aircraft:

The CG is in front of the CL.

Now imagine a gust of wind creating a pitching up force. This results in an increased lift (the angle of attack of the wing increases) that makes the lift force increase.

More lift creates a momentum to pitch DOWN! Pitching down decrease the lift, which brings us back to the original nose angle. This causes a balanced airplane.