Why do we experience shivering during winter when temperatures is very low?

Humans maintain a constant body temperature of 37 c.In summer, when outside temperature is more then our body temperature, we sweat profusely.The resulting evaporative cooling, similar to what happens with a desert cooler in operation, brings down the body temperature.In winter when the temperature is much lower than 37 c,we start to shiver , a kind of exercise which produces heat and raises the body temperature.