Different types of fossil.
Commonly people think of fossils as being the remains of bones and teeth. This has been highlighted by films such as Jurassic Park. This type of fossil contains the original bone or enamel of the animal but the tiny spaces within these structures have been invaded with hard chemicals such as calcite or silica and the proteins may be replaced, resulting in a discoloured and slightly altered version of the skeleton.
Petrified remains are also fossils. In this case the molecules of the original (usually 'soft' bodied) plant or animal, are replaced one by one with either carbon or silica. The result is a chemically changed replica. Carbonisation and compression results in the coal seams we mine (the burning of which retuns the trapped carbon to the atmosphere as carbon dioxide, adding to the greenhouse effect).
Casts are moulds of hollow bodies which became infiltrated with sedimentary materials. The sediment hardens and further reacts with silicates or iron oxide to preserve the interior shape of, for example a mollusc shell or the brain cavity of a larger animal. If the sedimentary material is fine enough the resulting imprint can be very detailed.
Fossilization is not to be confused with mummification where the original material is preserved by either extreme cold, chemicals or an anaerobic environment.