A fossil can be defined as any trace or artifact of life.
Fossils can give clues to the past and history of life on Earth.
Radiometric dating takes more probing of the fossil to find out the absolute age.
Relative dating relies on the law of superposition.
The dead organism needs to be buried under sediments and have a lot of pressure put on them.
The pressure cements the sediments together and then as they dry out, a sedimentary rock forms around the fossilized part.
While the fossil record is incomplete, it still helps lend evidence to the Theory of Evolution.