Here's a breakdown:

* Mold: This is an imprint left in the rock after an organism decays. The organism's shape is preserved as an empty space within the rock. Think of it like a negative image.

* Cast: If this mold gets filled with minerals, it creates a cast. The cast is a solid replica of the organism's shape.

So, a mold is the empty space, while a cast is the solid replica formed within that space.