Smouldering is a flameless form of  combustion. There are some fundamental differences between smouldering
and flaming combustion:

• Smouldering combustion is a slow, low-temperature process. Despite propogating an order of magnitude slower than a flaming fire and is only about half as hot it 'yields a substantially higher conversion of a fuel to toxic compounds'.  
• Smouldering occurs at the surface of the fuel and not at the gas phase as per a flame fire.

What smouldering materials have in common is that they are porous. This allows a free flow of oxygen to the oxidation front by diffusion and convection. From the diagram below you can see a differentiation between the oxidation and pyrolosis front. In effect the heat obtained through oxidation is feeding the pyrolosis and so the cycle continues until the fuel source is exhausted or the conditions are altered. The propogation of the smoulder is largely controlled by the rate of oxygen supply and heat losses at the oxidation front. 

The porosity of the fuel and the char (which becomes more porous due to decomposition) performs as an insulator, reducing heat losses thus sustaining the combustion cycle.