Decomposed remains of ancient plants and animals, which have been buried and heated and pressurized over millions of years. They come in forms of coal, oil. And natural gas. 

Nonrenewable resource: long to form. Cannot be replenished sustainably

Types: 

Coal: decayed plant region. Used in electricity generation and industrial processes

Oil: or petroleum. Tiny marine organisms. Refined into gasoline, diesel, used for transportation and heating and production of plastic

Natural gas: mostly methane, natural gas found in underground reservoirs. It heating, electricity generation.

How are fossil fuels used: burned to release energy. Power vehicles, generate electricity, heat homes, etc. 

When burned fossil fuels release co2, methane, nitrous oxide Fossil fuels is a big source of greenhouse gas emissions.