Double Pole Switch
The double pole switch also has "on" and "off" markings and functions similar to a single pole switch in that it turns something on and off from one location. However, because it has four brass terminals instead of two terminals it can handle switching two hot wires which allows it to switch a 240 volt circuit (or "220/221 whatever it takes..." if you're a Mr. Mom fan (1983)). The switch will also come with a ground terminal (green screw).
So bottom line, double pole switches are typically used to switch receptacles and appliances using 240 volt circuits.