Alpha, rated at 300hp or so, raw water pump inside, dog-clutch shifting, only works in one direction (need a reverse rotation model for twins left hand/right hand props...)
Bravo I, II, III, 400+ hp, raw water pump engine mounted, clone clutch shifting turns in either direction, heavier.
In case you were thinking about it, they are NOT interchangable. If you have an Alpha transom shield on your boat, you CAN NOT just put a Bravo drive on that transom shield. They may look similar, but they are not.