Fossils can be broadly divided into body fossils and trace fossils.

Body fossil – The remains of part (or all) of an actual organism.  In the kits, the trilobite (2), brachiopod (3), dinosaur bone (4), horse tooth (5), petrified wood (6), graptolite (7), fish (8), smooth and jagged-margined leaves (9 and 10), fern (11), shark tooth (12), cave bear tooth (15), and Spinosaurus tooth (16) are all body fossils. 

Synapsid trackTrace fossil – Trace fossils are traces or marks left behind by an organism. They show an organism was there, but are not the actual organism itself (which would be a body fossil). Dinosaur footprints and worm burrows are examples of trace fossils, as are coprolites (fossilized dung or feces). In the kits, the coprolite (1), theropod track (17), and synapsid track (18) are trace fossils.

Types of Preservation

Shark tooth

Unaltered hard parts - The body fossil is made up of exactly the same material as when it entered the rock record. Teeth are a good example as they are the hardest part in a vertebrate body, and they typically have the same composition in fossils as they do when they are in your mouth. Mummified fossils are also essentially unaltered.

Some mummified fossils include mammoths found frozen in the arctic permafrost, which are preserved by the cold just as a freezer preserves food.

In the Kits: #12: Shark tooth (specifically, the enamel of the tooth)

Brachiopod fossils

Recrystallization - A process by which the minerals making up the original shell or bone of a fossil change into a different mineral made of the same chemical components. Commonly, fossil shells made of aragonite will recrystallize into a more stable form of the same compound called calcite. Recrystallization preserves the shape of the original fossil, though sometimes fine details are lost as new crystals grow.

Sugar can be seen undergoing the process of recrystalization in substances like old honey or maple syrup.

In the Kits: #3: Brachiopod

Theropod fossils

Molds - Sometimes after an organism gets buried in rock, its original material can be completely dissolved by the groundwater flowing through it. What is left behind is a hole (or cavity) in the exact shape of the organism, which is called a mold.

This works he same was how if you cover a balloon with paper mache and then pop the balloon, its shape is still preserved by the paper mache.

In the Kits: #17 and 18: The tracks, sort of. (*The track preserves an impression of the foot by the same principle, but no one really calls a track a mold.)

Petrified wood

Permineralization/Petrification - This type of preservation normally happens with bone and wood, which have many tiny pore spaces inside. If you have ever looked inside a beef bone, you probably saw the marrow cavity, which is bubbly-looking, filled with tiny holes. These holes are the pore spaces. When groundwater seeps through the organism, it deposits minerals into these pore spaces. The minerals crystallize, hardening and preserving the organism. Permineralized wood and bone are much heavier in weight compared to the original organism, and they’re often discolored by the minerals carried in the water.

If you fill a sponge with saltwater and let it dry, the salt is left behind in the sponge's pore spaces. If the sponge broke down but the salt remained and held its shape, we would call that a permineralized fossil of the sponge.

In the Kits: #4: Dinosaur bone, # 6: Petrified wood

Jagged leaf fossil

Compression - These fossils are usually totally flattened (or squished) and two-dimensional, but still show most of the original shape of the organism. Leaves, stems, and soft-bodied organisms are often preserved as compressions.

You can preserve your own flowers with compression by pressing them in a book. Compression preserves a three-dimensional object in two dimensions, like squashing a balsa wood model.

In the Kits: #8: Fish, #9 and 10: Smooth and jagged leaves