Fossil fuels are the biggest source of energy for most countries. There are three major fossil fuels;

1. COAL - Its created from organic matter (forests) that were felled millions of years ago and were covered in sediment.  This mean they couldn't rot away and so were preserved. It burns slowly, and if its not in excess oxygen can be a dirty fuel as carbon or soot will be left over. Coal is usually extracted by mining, but is easy to extract the closer to the surface it is.
2. NATURAL GAS - Its formed at the same time as oil, its lighter so lies on top of the oil. Gas can be extracted using wells, that are dug down to find the reservoir. At the beginning, the gas will flow out due to the pressure its under, but as more gas is extracted pressure fluids have to be pumped down to maintain the pressure. The empty space left is filled with mud, so that any land above remains stable.
3. OIL - It was formed underneath the sea from the remains of microscopic sea creatures that died and fell to the ocean floor. They were then covered by layers of sediment, which trapped and compressed the organic remains. Heat and pressure slowly turn the organic content into oil. This percolates through the pores in the rock towards the surface, but if trapped by impermeable rock it forms an oil reservoir.

Fossil fuels are hydrocarbons, this means they are only made of hydrogen and carbon atoms. When they burn they produce water and carbon dioxide, but they also produce lots of energy, and this makes them useful.

Coal is used to fuel power stations, its burned and heats up water till it produces steam, which turns the turbines and produces electricity.