The heater blower fan motor is responsible for moving air through the car's cabin heating (and if fitted cooling) system. Much like a desk fan the heater blower motor rotates a fan which causes air to be forced through the ducting and out the vents in the car's dashboard or floor.
The heater blower fan motor is generally located behind the car's dashboard and is connected to a network of ducts which allow the fan to move air to various areas of the interior. The motor has a resistor which allows the speed to be varied via a switch or lever on the dashboard.
The heater blower fan motor can fail due to a bad connections, damaged wiring or switch, a blown fuse, a worn resistor, worn motor or because the fan is stuck or jammed against another internal component.