Fossils are the preserved remains of living organisms that existed on earth a long time ago. They are formed by the preservation of the remains of the organisms that existed in the earlier days. The dead bodies of the organisms are immediately covered by materials which do not allow decomposition at a fast rate. This preserves the dead bodies and forms them into fossils.
The existence of dinosaurs has come to be known only through fossils.
In addition to knowledge about the extinct forms, fossils also provide links between two groups indicating that perhaps one species evolved from the other. For example, a fossil called archaeopteryx show features of both reptiles (teeth) and birds (wings). Thus it shows some link between birds and reptiles.
The oldest fossil-bearing rocks contain fossils of few organisms that were simple. However, the younger rocks contain more fossils and these fossils show more complex structure indicating that the complex organisms originated from simpler organisms.
Fossils show changes in lifeforms over time. Fossils give us snapshots of life on earth at various times. We can trace the movement from simple plants like fungi and ferns, to more complex flowering plants, trees, etc. The development of animals can be similarly traced, from a time when reptiles dominated to the rise of mammals.
One of the best examples of evolutionary record (phylogeny) is of the horse. Almost the complete fossil record was found in North America.