The BiPAP device provides two different levels of air pressure: higher pressure when the person inhales and lower pressure when they exhale. This more closely mimics the natural breathing pattern, which can make therapy more comfortable for some people.
The BiLevel device or BiPAP provides two different pressure levels, one for inhalation and one for exhalation. Since the term BiPAP is registered by Philips Respironics, other brands prefer the generic term BiLevel. Basically, BiLevel and BiPAP represent the device working on the same principle.