Chemical reactions can be either exothermic (heat liberation or endothermic (heat absorption). Oxidation reactions are exothermic and reduction reactions are endothermic. Combustion is an oxidation reaction and hence is exothermic.
All liquid, solid, and gaseous fuels contain any one of the three combustibles constituents viz. carbon, hydrogen and sulphur. When a fuel is burnt the heat liberated makes the products of combustion hot.
The products of combustion are carbon dioxide, water vapour and sulphur dioxide. Along with these, the unused oxygen in the atmospheric air supplied for combustion and also the entire quantity of nitrogen which is the major constituent in the air are also heated. That is why fire is hot.
Source : The Hindu