Fossils are remains or impressions of (usually) extinct organisms. Fossils help to prove that these organisms existed. They also help to show how a species has changed over time.
The characteristics of fossils depends on the type of fossil. Mold fossils are impressions that are made in a substrate (often sedimentary rock). Trace fossils are like mold fossils in that the are impressions. However, trace fossils do not represent the organism itself. Instead, trace fossils represent an organism's daily life- such as footprints, nests, or burrows. Cast fossils are mold fossils that have been filled with deposits to form a 3-dimentional structure. If the casts, molds and trace fossils are made in sedimentary rock, then they may be very fragile and sedimentary rock is a softer rock when compared to metamorphic or igneous rocks.
An entire organism may be fossilized. For example, entire cavemen and woolly mammoths have been found frozen in ice. Other organisms, often insects, are found in entirety within amber. Amber is fossilized tree sap. These organisms would have been covered completely with the sap and fossilized over time.
Petrification occurs when minerals and sediments fill the pores of a dead organism and replace the tissues of the organism, thus replicating the organism. These fossils will be hard and not as fragile as a fossil formed in sedimentary rock.