Fossils form when sediments bury a recently dead organism.  The sediments
become rock, and the organism may be preserved as a fossil.

1) Petrified Fossils - the matter making up parts of the organism is dissolved
        by seeping water.  Minerals in the water replace the original matter.  The
        parts of the organism are turned to stone, but they still have the shape
        and details of the original organism.

        A)  Molds, Casts, Imprints - water seeps through the sedimentary rock,
        dissolving the hard parts of the organism.  There is a hole left in the rock 
        in the  shape of the organism. This is a mold.
        If minerals seep in later and fill in the mold - a cast is formed.
        A mold of a thin object is a imprint.
 
        B)  Trace fossils - footprints, tracks, trails, and burrows.

        C)  Coprolites - waste materials that have become petrified

2) Unchanged fossils - matter from the original organism has not been
           changed or removed.