The Shape Tween is similar to the Motion Tween with the exception that the Shape Tween can change the physical characteristics of your Tween. In Shape Tweening, you draw a shape in one Keyframe and then you change that shape or draw a new shape in the next Keyframe.
Making a Shape Tween
Step one: Draw a shape on the first Keyframe. For example a cloud.
Step two: Insert a second Keyframe, on the second keyframe you can use the subselect tool to highlight the anchorpoints and edit those, or delete your shape and draw in a new shape.
Step three: Click on a frame in the timeline, in your properties panel select Shape Tween from the pulldown menu.
Here is my example of a shape tween.