Buttermilk historically was the leftover liquid that Amish people would get after churning butter. Good stuff, you don't want to waste that! Commercially made buttermilk is acidized pasteurized milk. Buttermilk and sour milk are both highly acidic. Sour milk, while similar in flavor profile to buttermilk, has, historically, been milk that has undergone fermentation (milk that has not been pasteurized and has sit around too long, bacteria start doing their thing, and the PH level of the milk rises til it curdles)