Bi-paps and C-paps are both used for sleep apnea; some people, many with COPD, need the dual direction flow of the bi-pap to reduce C02 retention; others do just fine with one way pressure.

BiPap is actually a brand name.
Commonly known as a non-Invasive Ventilator NIV device used to help carbon dioxide from building up in the lungs due to low breathing pressures and lung damage. BI means that it moves air in both directions - In and Out. OUT is way more important with COPD.
The Cpap apnea device mostly forces air in one direction to prevent episodes of no breathing at all. The biPap device is programmable and adjusts to the breathing patterns during nightly use. It is often used in the hospital setting when treating a Flare-up episode.