Seriously? If you really don't know...it's the difference between pulling air from the outside vs. recirculating air from the vehicle cabin. I tend to use the latter because it keeps the exhaust and cigarette smoke smells out of my car from other drivers. Smokers are especially fond of hanging their cigarettes out of open windows and the smell always creeps into my car, but exhaust smell tends to make me sick to my stomach. I've also noticed when I put it on recirculated air, the air speed goes up. Maybe the car doesn't have to work as hard because it's not having to suck air from the outside?