Many kinds of things react with oxygen. The combustible substance of most candles is pure paraffin. It is a material that can be refined from crude oil. The candle's thread transports molten wax into the flame. Beeswax can also be used in candles. 

Wood is also a combustible substance. The combustion reaction of wood is similar to that of the candle discussed above. First, the wood is transformed into combustible wood gas, and then into water and carbon dioxide. However, what makes wood different from the candles is that even when all of the wood gas has burned out, there still remains small pieces of solid, combustible charcoal. In other words, wood burns twice: first as wood gas, then as charcoal. 

Wood is a popular fuel.

When the charcoal has burned out, what remains is a pool of ash.

Ash consists of minerals, which is why it is a useful fertilizer.

Ash contains all of the minerals the tree has collected from the ground during its life span.