Form of Fuel is a term used to describe the various sources of energy that are used to power machines and vehicles. The three most common forms of fuel are OIL, GAS, and PEAT. OIL is a liquid hydrocarbon that is extracted from the ground and refined for use in engines. GAS is a combustible mixture of hydrocarbons that is used in internal combustion engines. PEAT is an organic fuel that is formed from partially decomposed plant material and is burned as a source of energy.