The distinction between 'brushless DC' and 'brushless AC' is very simple:
BLAC: sinusoidal EMF. BLDC: trapezoidal EMF.
A BLAC motor requires a high-precision position feedback device (encoder or resolver), whereas brushless DC can get away with simple hall-effect sensors for position feedback - it's only necessary to know which 60 degree (electrical) segment the rotor is in.