Generally the term fossil fuel it used to mean the fuels produced from the remains of plants of animals, recovered from the ground.  The term usually excludes nuclear fuel although this is sometimes included.

Coal is the remains of plants mostly grown in the carbonaceous period which was between 340 million years ago and 280 million years ago.

Oil is probably formed from the remains of very small animals.  The conditions needed to form oil are very specific.  If it is too hot,

Natural Gas is formed.  Natural Gas is mainly Methane CH₄.  This is a colourless, odourless and colourless gas.

Methane Hydrate is a mineral composed of Water with Methane enclosed making a crystal structure.

Peat is what is produced on some wetlands.  It can be regarded as a very immature form of coal.