The three things that are required to start a fire are fuel, oxygen and heat, which form the fire triangle. All three are dependent on one another, which means that a fire cannot be started if even one of them is missing.
Oxygen in the air interacts with flammable vapors that are produced by fuel, whereby it creates a form of heat at a molecular level. After that, any source of ignition, such as a spark or a match, can cause it to combust. The more easily a material gives off flammable vapors when exposed to heat, the easier it is to start a fire.