Fire Indepth

Fire, a form of combustion, is a chemical reaction involving two or more chemicals where the molecules will readily react with each other to form additional chemicals.

Fire is combustion, what it is oxygen, fuel, and source that makes fire what it is(See Fire Triangle for more), The Fuel is what feeds the fire while the oxygen sustains the process, while oxygen and fuel alone dont do anything you need something to cause the fire, ie the source. The source is the energy which causes the spark you could say which igniates all these things into a exothermic chemical reaction we know as fire.  

Burning, or rapid oxidation, accompanied by release of energy in the form of heat and light. A basic cause of air pollution. 2. Refers to controlled burning of waste, in which heat chemically alters organic compounds, converting into stable inorganics such as carbon dioxide and water.

(Sited www.nsc.org/ehc/glossary.htm)