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."
Nowadays, as all milk and cream are pasteurized which kills the naturally occurring bacteria, buttermilk is made by reintroducing Lactobacillus acidophilus, the probiotic bacteria that gives buttermilk its sour taste