Fossils are our windows into the past. The word 'fossil' comes from the Latin word fossus, which means 'dug up'. This refers to the fact that fossils are the remains of past life preserved in rock, soil or amber. Generally, the remains were once the hard parts of an organism, such as bones and shell, although, under exceptional circumstances, soft tissue has also been fossilised. There are different types of fossils because remains can be preserved in a variety of ways. These include trace fossils, fossils with some organic material preserved, mineralised fossils and impression fossils.
Sources: http://australianmuseum.net.au/what-are-fossils