Additional info to 737aviator's response: For the stretch versions (-400, -800, -900) there are two recirculation fans while the non-stretch aircraft use one. The fan intake air is filtered. During normal operation no recirculated air comes forward to the flight deck ducts, fans on or off.
We discovered on hot days at the gate out west, particularly after a long period on stand, preconditioned air seems to cool better with the RECIRC fan(s) AUTO just as 737aviator noted above. The fan(s) give the illusion of more cooling by increasing the airflow, but in fact the temperature of that airflow can actually go up when the fan(s) are on during ground operations. In humid environments back east on a quick turnaround, RECIRC OFF seems to be the better configuration until off stand with both packs running.