I've heard it said that what we buy as buttermilk on the supermarkets now isn't the same as what buttermilk used to be.  Can you explain the difference to me?
--Strong

Back in the day, as my kids would say, buttermilk was the liquid that was left behind when butter was made.  The milk would be left stand for the cream to rise, and perhaps longer until enough cream was collected to churn.  During that time, the milk or cream would ferment naturally.  Once the butter was removed by churning the left over liquid, or butter milk, would continue to thicken.

Now buttermilk is plain milk, usually skim or low fat, that has been treated to make it into buttermilk.  With the advent of centrifugal cream separation the process of making butter could be sped up allowing more butter to be made in a shorter time.  Thus, the natural fermentation that occurred during gravity separation was short-circuited.  The resulting skim milk is heated and then inoculated with lactococcus and leuconostoc bacteria and left to ferment.

Buttermilk made by the old process is referred to as traditional buttermilk, while the newer process makes cultured buttermilk.

By the way, this also explains why traditionally made butters have a somewhat more tangy flavor, since they would carry some of the fermentation by-products that occurred during the cream separation.