Coal and Oil are both fossil fuels. Please can you tell me what the main difference is in the way they are formed. Thank You.

Check these resources:

How oil is formed from Goff Petroleum.

Forming oil from "How Stuff Works."

How coal is formed from the American Coal Foundation.

For an explanation of the difference:

How Fossil Fuel is Formed.

Oil and Coal (and Natural Gas) were formed millions to hundreds of millions years ago from the decomposed remains of plants, trees, and/or animals that were compressed and heated by overlying sediments of mud and rock over millions of years. Oil was formed from the compressed remains of early ocean animal (marine organisms) and plant life, while coal was formed from decomposed plants and trees only, beneath former freshwater and ocean swamps. Coal is primarily compressed organic peat. Both were formed millions of years before the appearance of the dinosaurs.