Fossils are divided into two groups-- Body Fossils and Trace Fossils. Body fossils are the actual fossilized remains of the animal or plant (such as bones or leaves) whereas Trace fossils record the activity of an animal (such as footprints, trails or burrows.)
The Body Fossils are further subdivided into three groups:
Mold and Cast body fossils (such as dinosaur skeletons)
A cast or a mold fossil is an impression of a living organism. They are made when an organism dissolves in the Earth and leaves a hollow mold behind. The mold is then filled in by minerals leaving a solid shape of the organism.
Replacement body fossils (such as petrified wood)
Replacement fossils occur when the organism is buried by sediment and thus protected from decay by oxygen. In the example of petrified wood, groundwater full of minerals flows through the sediment over time replacing the original plant material with silica, calcite, pyrite or another inorganic material such as opal. The result is a fossil of the original woody material that often exhibits preserved details of the bark and cellular structures.
Whole body fossils (such as mammoths caught in ice or insects trapped in amber) Whole body fossils preserve the actual cellular tissue of the animal.