Paleontology is key to the study of evolution for two reasons.
What is a fossil? Generally, we think of bones, shells, or teeth that are buried in rock, but fossils can also be outlines of leaves or footprints or trails. This second set of fossils, which are the outlines of items from the past rather than the items themselves, are called trace fossils. Fossils are formed when sediment covers some material, such as a piece of bone. Very gradually, the bone becomes impregnated with chemicals from the surrounding rock. Eventually all that remains is essentially a piece of rock in the shape of the original bone, or material.
Taken together, fossils can be used to construct a fossil record, which is a timeline of fossils reaching back through history. Several factors must be taken into account when constructing such a record. The strata of rock in which fossils are found give us clues about their relative ages. Similarly, new technological techniques such as radioactive carbon dating help determine the absolute ages of fossils. In addetion to supplying a fossil's relative age, rock strata can also give clues about the environments in which an animal or plant lived. The chemical make-up of these strata can tell us the balance of gases in ancient atmospheres. Major cataclysmic events such as eruptions and meteor strikes also leave there mark on the fossil record.
There are, however, limitations on the information fossils can supply. Fossilization is an improbable event. Most often, bones and other materials are crushed or consumed before they can be fossilized. In addition, fossils can only form in areas where sedimentary rock is formed, such as ocean floors. Organisms that live in these environments are therefore more likely to be fossilized. Erosion of exposed rock faces or through the crushing action of geological movements can destroy fossils even after they are formed. All of these conditions lead to large and numerous gaps in the fossil record.
The Brief History of Life provides, as you might expect, a synopsis of the history of life that has been determined through fossil records.