From the left to the right we have 3 different Shape Tweens:
Shape hints
Two shape hints were added to the Keyframes at Frame #1 and Frame #25. Notice how the Square morphs CLOCKWISE to become a Circle and the Circle morphs CLOCKWISE to return to being a SQUARE form #25 to #50.
Ease In - Ease Out
The Square Eases In to being a Circle and then the Circle Eases Out to become a Square again.
Shape Tween
Absolutely basic with no shape hints and no easing in or easing out, so a constant rate of change from
one shape to the other. Not customized at all.
Shape Tweening is MORPHing and first became widely known via the Terminator movies. California governor Arnold Schwarzenegger MORPHed his career from movie magic to politics.