Hello!
Here is the short story behind CPAP vs. BiPAP -
CPAP (or CONTINUOUS Positive Airway Pressure) is a unit that delivers ONE set pressure (generally, 4-20 cms of H2O) through a tube into an interface (mask) into your airway. This one pressure will eliminate up to 90% of apnea/hypopnea events in someone with Obstructive Sleep Apnea.
APAP (or AUTOMATIC Positive Airway Pressure) is a unit that automatically adjusts your pressure throughout the night to try and achieve the OPTIMUM pressure at each stage of sleep. Like CPAP is uses one pressure, but it continually adjusts this one pressure automatically to assure you are receiving the minimum pressure to aleviate apneas and hypopneas.
BiPAP/Bi-Levels (or Bi-LEVEL Positive Airway Pressure) is a more complex unit that bases therapy on TWO pressures, one pressure for inhalation and one for exhalation (generally between 4-25 cms of H2O). These two pressures work together to ensure that your are receiving pressures appropriate for inhale and for exhale. Many different conditions/types of breathing disorders require BiPAP - it is NOT a top-of-the-line CPAP, it is an entirely different form of therapy and if you need one, it will be prescribed specifically by your doc.
BiPAP ST or Bi-Level ST is the same unit as described above, but it also has a "back-up ventilation rate" that can actually work as a non-invasive ventilator for people in respiratory distress.
I HTH! Good luck!