Buttermilk is the liquid left over when making butter from milk, thus the name. While it's not exactly a health drink, buttermilk is still a better alternative than some other drinks on the market.
The methods for making buttermilk have changed over the years. In the old days, buttermilk was fermented by naturally occurring bacteria as unpasteurized butter cream sat for a length of time before churning. This process is called "ripening."