A fuel is defined as any substance, solid, liquid or gas which can be easily ignited and burned to produce heat, light or other useful form of energy. For example, coal, charcoal, gasoline (petrol), kerosene (paraffin), light oils, fuel oils, natural gas, liquified petroleum gases, hydrogene, etc.
The oxidizer is a substance that will react with, and promote oxidation of a fuel. Oxygen is the most common oxidizing agent involved in combustion (burning) processes.
The oxygen supply for combustion usually comes from the air, because obtaining pure oxygen is expensive and difficult. Because air contains a large proportion of nitrogen, the required volume of air is much greater than the required volume of pure oxygen.