The principle of follow through demonstrates that in order to make an animation move realistically, loose objects should continue to move after the initial movement has stopped. This backs up physics of an animation as well as preventing immediate standstills of an animation occurring which all around appear unrealistic and boring when done.
Overlapping Action describes the principle of simultaneously moving objects differing in speed/rate from each other. In terms of body parts, someones head and arm moving at the same time but at different speeds.
Wikipedia notes Drag as a related technique in which objects take a few secodns longer to react to an initial movement of a character – eg. Clothing dragging behind a runner as it catches in the wind and very rarely travels at the same pace as the runner.
The first image demonstrates Follow-Through with the hair continuing to move as a direct result of the head movement that stops seconds before.
The second image demonstrates Overlapping Action showing bother the leg movement and the cape travelling at different speeds to one another.