Follow Through and Overlapping action occurs in real life, not just animation.
When the main mass of a character stops other parts, such as the hair, clothing etc continue moving (following the path of action). The follow through is created as nothing stops at one time.
Overlapping Action is when a character changes direction while these secondary items are moving forward. The character changes direction and a few frames later by these items.
Drag is another thing considered in these thing. Drag is the frictional force acting in the opposite direction to the motion being done, the greater the drag the greater the force needed to create the movement.
Video courtesy of AnimatorIslandTV.