There are two kinds of buttermilk:
Old-fashioned buttermilk is the liquid that separates from the solid butter after churning. It looks like skim milk but it may be a bit sour, depending on the type of butter made.
You get cultured buttermilk when you add a culture -a lactic acid producing bacteria- to fresh milk and allow it to ferment. It's almost the same process as when making yogurt, and the resulting buttermilk is very similar in taste.
Buttermilk can be enjoyed straight from the bottle, or it can be used to make pancakes, biscuits and scones. If you want to produce moist-yet-not-too-heavy Irish soda bread then you need buttermilk.
Buttermilk makes a good starter when making soft farmhouse cheese. Cultured buttermilk can substitute yogurt in marinades and savory dishes .