The most informative fossils preserve the actual remains of an organism. For instance, the skeleton of a vertebrate may be fossilized intact. This occurs when minerals seep into the pores of the bone and petrify it.
The bones are no longer composed of the same elements and minerals as they were when the animal was living. However, they still have a lot of information to provide.
Trace fossils are the preserved impressions of animals or plants, similar to the impression of your hand in clay or cement. The organism decomposes in water, but an imprint of it is rapidly filled by sediment, preserving it indefinitely. More information has been lost in this process, but a cast of the organism is left behind.
Carbon acts to preserve an impression of an organism, though usually one more delicate than a skeleton: a leaf or fragile animal.
Over time, pressure from the fossil’s rock eliminates liquid and gas, leaving behind carbon. The carbon leaves a shadow of the surface of the organism, sometimes in great detail.
Another kind of fossil is a whole organism trapped in fossilized tree sap (amber). Insects and other small organisms such as lizards or spiders were sometimes trapped in tree sap that later hardened.
The jewel-like fossils reveal an actual organism millions of years old.
Ice is another preserver, if left undisturbed. Nearly intact human (as well as animal) remains have been discovered in ice.