Coal is a fossil fuel which is as hard as a stone. It is black in colour and is mainly used as fuel.
As coal is a fossil fuel, it was formed by the dead remains of the plants. Millions of years ago vegetation in forests got trapped under soil layers due to earthquakes and volcanic eruptions. More soil layers got deposited on it and it went deeper and deeper beneath the earth. This raised the temperature and pressure beneath the soil. This initiated the process of burning these dead remains. Slowly all the vegetations got converted into coal which is a rich form of carbon. This whole process is also called carbonization.