Fossils are remaining parts of organisms found in deep Earth's crust who died million years ago.

Formation of the fossil is a very complicated yet interesting process. When an organism dies due to any reason(may be some calamity like earthquake or tsunami or flood or even natural death), its body gets buried inside the earth's crust or even deep sea level. The soft tissues of that organism get decomposed by various bacteria or other organisms but its hard tissues like bones/teeth etc remained un-degraded. Moreover, due to the high pressure inside the crust, they become more hard and tough. Over a long period of time, a lot of layers will get formed over them and they will get deeper & deeper. During paleological researches when these layers are deep drugged, these remnants were discovered in the form of fossils.

So this is how fossils are formed.