Shapes are two dimensional and geometric. When you draw a circle, square, or triangle you are drawing a shape. Forms are three dimensional and geometric. These include cylinders, pyramids, and cones. I would say that the most prevailing shape in bonsai is conical.
Besides the obvious silhouette made by a form they are important in that they can add to motion. In sculpting rhythm is important. Adding forms to sculpture gives rhythm. Repetition of shapes in varying sizes creates rhythm.
|The trunk and foliage are two different cones|
Shapes and forms are easier to understand than some other elements. The key is to use them to guide the eye. Cheers.