People have different definitions of
what fossils are, but basically the key point is that fossils are some trace of an ancient extinct organism. Most of the fossils, especially when
it comes to dinosaurs, are bones like these big vertebra or teeth from a dinosaur, hard parts. And, if you're looking for other kinds of
fossils, seashell fossils of various kinds, there are fossil leaves in wood from plants, all of
those qualify as fossils because they're traces of ancient organisms.
Sometimes, other types of body parts like skin and
fossil footprints that were left by animals are also found, and these too
qualify for fossils. So basically, a fossil is any trace of
an ancient organism, either a hard part, or a soft part, or some product that they
left behind and usually this trace has to be over ten
thousand years old in order to be considered a fossil.