Combustion happens when a material is exposed to oxygen under high temperatures. This material is known as fuel. Many elements can be used as fuel, such as kerosene, sodium and cotton. Compounds such as wood (tissue) and coal are also easy to burn. It happens as follows:
Fuel + Oxygen -> Flame -> Water + Ashgas + Ash
It is worth noting that the water produced is minuscule in amount.
The very reaction is usually triggered by heat, but it produces heat too. This means it can expand very quickly if the ingredients are abundant.
The ashgas immediately reacts and binds with the water creating carbonic acid.