Buttermilk is the liquid left over after churning butter. It usually has a sour flavor due to controlled fermentation.
Because most of the fat is removed with the butter, buttermilk is not very rich. If derived from naturally-cultured cream, the buttermilk has a naturally sour taste; if derived from sweet cream, the buttermilk must be further fermented to get its typical sour flavor. Cultured buttermilk is fermented with lactic acid bacteria, much like yoghurt, giving it a tart taste.
Buttermilk contains calcium.