CPAP --> Constant Positive Airway Pressure (one pressure level, used all the time)
BiPAP --> Bi-level Positive Airway Pressure (two pressure settings -- one used during inhalation, one used the rest of the time).
Either therapy can be and is used with both nasal and full-face masks.
Yes, you do need to be able to breathe on your own to use them effectively. Even though some BiPAPs have the ability to "push" a breath after a certain length of time has elapsed since the last breath (called S/T, or spontaneous/timed, mode), they are not replacements for full-fledged ventilators.
CPAPs are not recommended for use in patients who have hypoventilation due to neuromuscular weakness caused by ALS.
Hope this helps.