The case itself will usually have a fan to move the air from the PC case out through the system. The motherboard, on the other hand, has multiple fans to do this. The case fans are normally connected to the motherboard with a fan connector, which has multiple pins. Each of these fans has its own control circuit, which is usually connected to the mainboard with a 4-pin fan connector. The mainboard has a control circuit to control the speed of these fans, based on the cooling requirements of each component.