A good fuel must be readily available, must be a good conductor of heat energy, must be easy to store and must not produce any toxic gasses. A good fuel must also be available for purchased at a relatively inexpensive price.
Heat, oxygen and fuel are necessary for a fire to burn in any setting. Heat must be used initially, oxygen then increases the heat into a small spark or flame, and finally fuel allows the heat to continue burning. Fuel is used in the chemical process of combustion. Combustion is one of the easiest and most readily available ways heat can be produced. It is done via exothermic reaction. Exothermic means that the product of the chemical reaction is released outside of the reaction. Fuel does not necessarily have to be extremely combustible but must provide a source of constant heat.
Wildfires and forest fires are often subject to fuels. These can include roots, trees and brush. The dryer the fuel, the more able it is to produce a large amount of heat. Oxygen is readily available from the surrounding air, during forest fires and this gas works in combination with the fuel to keep the fire burning.