A single stage snow blower has just one auger to scoop and throw the snow out of the way, whereas a two stage has two sets of augers: one to gather snow into the tub and one to throw it off to the side. Having two sets of augers means the snow blower will have more power when clearing snow, so they're well-suited for areas that get heavy snowfall every year or deal with wet snow fairly often. Single stage models are weaker, so they're best suited for light snowfall or powdery snow.