What are the comparisons between soy milk and cow's milk?

It isn't, necessarily. Soy milk is derived from soy beans (a plant), so soy milk is free of lactose, a protein which is found in cow milk, and also contains much lower levels of saturated fats and cholesterol. On its own, soy milk does not contain much calcium, so this is generally added by manufacturers. Soy milk contains hormones that mimic the action of oestrogen in the human body which may have some health complications for some.