Center of Gravity
Every object has a center of gravity, or CG. The CG of an object is an imaginary point where all of the mass of that object can be considered to be concentrated. An object will rotate around its center of gravity. For example, a hammer, if thrown, will rotate around a point very close to the hammer's head, where most of the mass of the hammer is concentrated.
The CG is also the balancing point of an airplane. If you wanted to balance an airplane on the tip of your finger, if that were possible, you would have to lift the airplane at its CG in order for it to balance there.
An airplane is considered to move about its CG on three axes of motion - longitudinal, lateral, and vertical. Axes are imaginary lines that pass through and are connected at the CG.
Longitudinal Axis
The longitudinal axis runs through the airplane's CG from nose to tail. Movement about this axis is referred to as roll. Roll is the left and right rolling motion of the airplane that occurs when the wings are raised and lowered.
Lateral Axis
The lateral axis runs through the airplane's CG from wingtip to wingtip. Movement about the lateral axis is referred to as pitch. Pitch is a nose up or nose down movement.
Vertical Axis
The vertical axis runs through the airplane's CG from top to bottom, vertically. Movement about the vertical axis is referred to as yaw. Yaw is the left and right movement of the nose.