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"Body" fossils include bones, eggs, claws, teeth and even skin and muscles of ancient animals. Bones and other hard parts of animals fossilize better than skins, muscles and other soft tissues, but certain conditions allow these softer parts to be preserved. For example, an insect caught in amber can become perfectly preserved, as well as mammoths preserved in ice.
On the other hand, "trace" fossils, also known as "ichnofossils," are any trace of, or evidence about, the movements and habits of ancient life-forms. This includes animal dung, shelters, burrows, nests and even imprinted tracks.
Mold Fossils
"Mold" fossils form when plants or animals decay under a layer of sediment and rock. As time goes by, the remains of the life form "imprints" itself on the rock, creating a hollow impression of what the life-form looked like. There are also times when the hollowed space created by the remains gets filled by minerals and other substances, creating a "cast" of the life form from the mold. When this happens the fossil is labeled as a "cast" fossil.
True Form Fossils
As the name suggests, "true form" fossils are body fossils preserved in an almost perfect condition, allowing us to see and study their complete and original form. Animals and plants preserved in ice, sap and amber can be examples of true form fossils.
Petrified Fossils
"Petrified" fossils are plant matter or animal tissue gradually replaced by minerals, such as quartz. Petrifaction happens when these life-forms get buried in sediment and rocks, delaying its decomposition by limiting oxygen and other substances required for decomposition. Over time, water containing minerals penetrates the sediment, slowly replacing the parts of the life-form with minerals.