A material is combustible if it can be used as fuel to release energy in the chemical combustion reaction also known as burning.
We must provide heat and oxygen to the fuel for this chemical reaction to occur. Without one of these ingredients, the combustion will become incomplete and eventually die out. A good example of combustion is a log fire, where the wood acts as the fuel, and the heat from the flame is the energy released in the reaction.
Flammability describes the difficulty of using a material as fuel for combustion. A substance is highly flammable if it can be ignited with ease, with a single spark, or at low temperatures like paper or hydrogen gas. These substances should be handled with care.
Figure 1 - Combustion requires oxygen, fuel, and heat.