The exothermic reaction is the opposite of an endothermic reaction. It releases energy by light or heat to its surrounding. A few examples are neutralisation, burning a substance, reactions of fuels, deposition of dry ice, respiration, solution of sulphuric acid into water and much more.
CH4(g) + 2O2(g) ----> CO2(g) + 2H2O(l)
CaO(s) + H2O (l) ----> Ca(OH)2(s)