Fossils are the evidence of past life that has been preserved in rocks.

As such, fossils can cover wide range of features. As well as the relatively common 'hard preservation' of common shells, they can include organic carbon films (e.g. leaves); isotope deviations (e.g. the hints of early ~3.5-4Ga life); chemical fossils (amino acids, keratin, breakdown products); casts (eg. at Pompeii); footprints; coprolites (fossil dung); regurgites (fossil vommit); burrows; and even buried cars, telephone boxes, etc ('technofossils') could be considered fossils.