Cast fossils:
Cast fossils form when a mold is filled in. Once a hollow mold is present, they are filled in with minerals that form a solid.
Mold fossils:
Mold fossils are an imprint mark made in a rock or sediment. They are hollow, and are often leaves, teeth, claws, and more.
True Form fossils:
True Form fossils are big parts of an organism went through a process called petrification, in which their body part was replaced by minerals which hardened to become rock.