The three main forms of fossil fuels are coal, oil and natural gas. These three are also known by different names, such as petroleum for oil, and each type of fossil fuels exists in a different chemical state.
Coal has been used for centuries for heat and to power steam engines. It is the only main fossil fuel that is in a solid state. Oil, or crude petroleum, exists in a liquid state and is refined to make gasoline. Natural gas is primarily made of methane gas. Each of the three fossil fuels require different refining methods to make them useful to humans.