The ventilation fan exhaust outlet is on the back of the fan for wall mounted models and on the side for models that are ceiling mounted. The exhaust outlet is where the air pulled from the room is discharged into the duct work, or a dedicated vent, and channeled outside.
For example, looking at Panasonic's line up of ventilation fans, the Whisper Green 80 CFM Ventilation Fan is mounted on the ceiling, but the exhaust outlet is on the side. The fitting allows the fan to be integrated into the home's ventilation system, though if there is direct access to the roof, its own vent can be installed via an elbow joint.
As a contrast, the Whisper Wall Mounted Fan has its exhaust outlet on the back. By using an elbow joint, the fan may also be connected in the same fashion as the ceiling fan. This fan works best in areas where a ceiling fan is impractical, such as in a multi-story apartment building or some vintage home restorations.
Both fans use back draft dampers to help pull air into the device for discharge, which also prevents outside air from entering the home. Ventilation fans must always be vented to the roof or an outside wall. Discharging warm damp air into an attic or crawl space encourages mold growth, which can be toxic.