With a CPAP machine, the pressure of the air delivered to the patient is continuous. This can sometimes make it a little uncomfortable for the wearer of the device to exhale, as he or she is actually exhaling against inbound pressure.
With a bilevel positive airway pressure (BiPAP) machine, also known as a BiLevel or VPAP machine, two levels of pressure are delivered -- one at a higher level when the patient inhales, and another at a lower pressure when he or she exhales. Just as with a CPAP machine, the levels of pressure are prescribed by a physician. Because of the reduced effort to exhale, BiPAP machines are often more comfortable for the wearer and tend to lead to fewer side effects like sore throat.
Both BiPAP and CPAP machines are helpful in eliminating central and obstructive sleep apnea.