Back in the day, unpasteurized cream was left on the counter in a covered container to ferment for a few days; it was then churned to make butter. The liquid left behind after the butter was removed was buttermilk. Today, it's made by adding lactic acid bacteria to low fat or skim milk. Buttermilk adds a nice tang to soups, salad dressing, and baked goods, and can be substituted one for one in a recipe. Add 1/2 teaspoon baking soda to the dry ingredients for every cup of buttermilk.
Just note that buttermilk will separate if brought to a near boil, so be selective when substituting.
In a pinch, make your own buttermilk- mix 1/2 cup plain yogurt with 1/2 cup milk, or stir 1 tablespoon of lemon juice or distilled vinegar into 1 cup of milk and allow it to stand for a few minutes.