What are fossils?
Fossils are a snapshot into the past, in some cases the very distant past.
They are the remains or impressions of prehistoric life preserved in petrified form or as a mold or cast in rock, they can be as much as 3.5 Billion years old or as little as 10,000 years old and they tell us an interesting story of the Earth and its days gone by.
Fossilization occurs when a dinosaur, for example, dies and lays out in a river bed or a water catchment area and is at some point covered in water. Sediments and silt then cover the creature and over many thousands or millions of years these sediments build up and build up until eventually they become a sedimentary rock that has fully encased the dinosaur.  After many years of rain and water seeping through the rock the bones are eventually replaced by minerals and silica and turn into stone or petrify.
With a combination of Earth movement and weather erosion, parts of the bones can become exposed for us to find and excavate.
There are two kinds of fossils;
1. Petrified fossils are the bones that we dig up of dinosaurs, ammonites and teeth etc..but without any other impression of what they might of looked like.
2. Cast or mold fossils which are often fish, although there are some dinosaur fossils that are mold type fossils
(archeopteryx is one).  Mold fossils are when the flesh of the animal has rotted away and left an imprint of itself in the rock with perfect clarity as to what it looked like, if it had feathers or skin as well as its bone structure.