Paleontologists (scientists who study fossils) tell us there are two main types of fossils: body parts fossils and trace fossils.
Under normal circumstances, plants or animals that die do not last very long. Scavengers might eat the body, or perhaps it will rot from bacteria that cause it to decompose. Just exposure to air and sunlight can also cause the organism to disintegrate. So, how did it happen that billions and billions of things died and are preserved for us in the rock layers of the earth? The answer lies in how fossils were formed.
There are different kinds of fossils, but most all have three things in common. They all involve death, quick burial (which closes them off from other things), and large amounts of wet sediment. Fossils do not require millions of years to form. They do require exactly the right conditions.