The term 'fossil' is derived from a Latin word meaning 'dug up', and from the time of the ancient Greeks it was used to describe any distinctive objects or materials dug up from the earth or found lying on the surface, including minerals, ores, rocks and stone implements, as well as organic remains. It was not until the nineteenth century that the word became restricted to the remains or traces of pre-existing life preserved in rocks. Applied in this sense, it refers not only to the preserved remains of organisms (eg shells, skeletons, leaves, tree trunks) but also to their impressions in the rock, as well as to tracks or trails formed by organisms when alive.

Fossils are formed when a living organism dies and the body or part of the body is preserved in some way, usually, but not always, by being rapidly buried in sediment. Burial prevents destruction of the organism by scavengers, bacteria, or weathering. Burial can occur on the bottom of the sea or in rivers or lakes, or it can occur on land by blown sand, falling volcanic ash, or by the entrapment of the organism in some sticky substance such as tar (e.g. La Brea tar pits in Los Angeles) or tree sap (insects in amber).

Very few individual organisms are preserved out of the billions that have lived throughout geological time. Usually only hard parts (shells or skeletons) are preserved; however, in exceptional circumstances soft parts can also be preserved, such as mammoths in the tundra of Siberia or the mummified flesh of moas in some New Zealand caves.

The fossilised bodies or hard parts of animals and plants are usually called 'body fossils' to distinguish them from 'trace fossils', which represent the signs or remains of an animal's activities. Footprints, trackways, bite marks, borings, feeding and dwelling burrows, and droppings (coprolites) are all regarded as trace fossils.

Very fine grained sediments can preserve details of organisms very well. For instance in the Burgess Shale of British Columbia, Canada, of Middle Cambrian age (517-536 million years old), fossils of many kinds of animals complete with all their delicate appendages are preserved as mineralised films. The Koonwarra fossil bed of South Gippsland (Early Cretaceous, approximately 118 million years old) has insects which still show signs of the original colour patterns. After burial, fossils may undergo a variety of physical and chemical changes depending on the environment within the enclosing sediment. The physical changes may involve the fossils being flattened or distorted as the sediment is hardened into rock (lithification) by intense heat and pressure in the Earth's crust. The fossils may be changed chemically by having the original hard parts progressively replaced by another mineral such as gypsum, pyrite, phosphate, quartz or opal, a process called petrification. Or the hard parts may be dissolved away entirely, so that only an impression or natural mould of them remains in the rock.

Geological objects of inorganic origin, such as distinctively shaped rocks or minerals, are sometimes mistakenly interpreted as fossils owing to the power of the human imagination. Such objects, called pseudofossils, do not deceive only the inexperienced person; sometimes professional palaeontologists have been misled. Pseudofossils have a variety of origins. Some are sedimentary structures formed by the action of water currents during deposition (e.g. ripple marks, impressions made by pebbles or other objects transported by currents), or by the drying out of soft sediments exposed to the air (e.g. mud cracks, clay curls). Others are rounded objects formed by deposition of some mineral around a nucleus during the process of rock formation (concretions or nodules). Pseudofossils may also be formed by the growth of minerals, sometimes as radiating crystals and sometimes as irregular branching growths (manganese dendrites, commonly mistaken for fossil ferns, mosses or other plants).

Information provided by: http://www.museum.vic.gov.au