I found this brief description on the web. There are some descriptions a whole lot more complicated. I'm not sure I really understand it either, but I didn't do well with the BiPAP. Or I guess it can be written as Bipap.
Cpap Machine
A Cpap machine provides continuous air pressure at a medically suggested level to treat various types of sleep apnea. Because the air is continually flowing from a mask, it forces the throat to stay open to prevent episodes of sleep apnea.
Bipap Machine
A Bipap machine has different levels of pressure during inhalation and exhalation. This makes it easier for people to adjust to the machine. Because of its easier application, a Bipap is often a better method for people suffering from congestive heart failure, neuromuscular diseases or various lung disorders.