The drivers side (assuming your not in the UK) fan is driven by the fan/alternator/A/C compressor belt. It runs all the time the engine is running. Don't stick your finger in it, you never know when the clutch has failed. Picking your fingers up off the pavement is a rough way to find out the clutch is locked up. The passenger side fan is driven by an electric motor and only comes on when the A/c compressor is engaged to remove the extra heat added to the coolant caused by running the compressor, and the heat discharged from the A/C condenser infront of the radiator.