What is the difference between a CPAP and BiPAP machine?

Positive airway pressure (PAP) is often used in hospital settings to assist patients’ breathing. The patient wears a mask attached to a machine that helps keep the lungs and airway open and expanded, improving the lungs’ ability to transfer carbon dioxide and oxygen. PAP is well-suited for patients who do not require intubation and ventilation but are experiencing a certain degree of respiratory distress.
CPAP (continuous positive airway pressure) and BiPAP (bi-level positive airway pressure) are typically used to treat sleep apnea; however CPAP is also used to help premature babies with under-developed lungs. CPAP helps people with sleep apnea by using pressure to keep the airways open, preventing the collapse or obstruction that causes snoring and potentially dangerous pauses in breathing during sleep.
Unlike CPAP which creates constant pressure, BiPAP offers two levels of pressure for patients who cannot tolerate exhaling against the level of pressure indicated for their condition. BiPAP temporarily reduces the level of pressure at the start of exhalation then increases it back to the indicated level prior to the next inhalation.
Both CPAP and BiPAP machines are adjusted to exact specifications in order to correct each individual patient’s level of breathing disorder, which, in the case of sleep apnea, is determined during a sleep study.