Exhaust-only or spot ventilation systems are strategically placed exhaust fans that remove moist and polluted air. The fresh air either comes from open doors or windows or random leaks unless you have a tight house, where vents that passively open with negative air pressure can provide the needed fresh air.
Supply systems use forced-air heating or air-conditioning systems to supply fresh outdoor air through existing ductwork. These systems involve control mechanisms, and do not necessarily reduce energy consumption. Diligent use of exhaust fans is still needed.
Balanced systems capture the heat from exhaust air to condition the supply air, capturing the majority of otherwise lost energy.
The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) recommends that mechanical ventilation be no less than 50 cubic feet per minute (cfm) with 50 to 100 cfm requirements for kitchens and bathrooms.