What are fossils? What do they tell us about the process of evolution?

Fossils are the dead remains of plants and animals which undergo several physical and chemical processes after they are dead.

Example- Archaeopteryx lithographica is a fossil that resembles a bird (has feathers and wings) but has several features of reptiles (tail and teeth). So Archaeopteryx is supposed to be the connecting link between ave and reptiles thus suggesting that birds have evolved from reptiles