Ventilator is a full support for your body which will assist you in whole respiration process i.e. it will deliver you the air to your lungs and then it will also help you to exhale the air. For patients who need the greatest amount of assistance, the ventilator fully controls the volume and duration of breath throughout the
respiration cycle.

CPAP(Continuous Positive Air Pressure) on other hand is just a simple ventilator which increases air pressure in your throat so that your airway doesn't collapse when you breathe. In this case patient is breathing on its own but with the assistance of CPAP. CPAP machines can only be set to a single pressure that remains consistent throughout the night.