Continuous Positive Airway Pressure (CPAP) devices or modes apply constant pressure throughout the respiratory cycle via face mask or other interface to splint open the upper airway, increase lung volume, and increase intrathoracic pressure. CPAP provides no inspiratory muscle unloading and tidal ventilation remains completely dependent on the respiratory muscles.
Non-invasive ventilation (NIV) or Non-invasive positive pressure ventilation (NIPPV) applies two levels of pressure during the respiratory cycle – a pressure during the inspiratory phase that is greater than the pressure applied during exhalation.  This is effectively mechanical ventilation, and can unload the respiratory muscles and provide complete respiratory support.
Bilevel positive airway pressure (BIPAP) is a branded/trade name (by Phillips) for NIPPV/NIV as described above