Real buttermilk, the stuff your grandparents may remember seeking out for refreshment on a hot summer day, is what is left after you've turned cream into butter. It is smooth, rich, and delicious. It is rarely for sale, and you need to make it yourself by making homemade butter.
The cultured buttermilk for sale at most grocery stores, however, is a cultured milk product unrelated to the making of butter and can be easily and quickly made at home when a recipe (like this Buttermilk Cake) calls for it. Simply add 1 Tbsp. white vinegar or lemon juice to 1 cup of milk. Stir or shake and let sit at room temperature for 5 to 10 minutes. The acid will thicken the milk a bit and you'll have a fake buttermilk that works perfectly for cooking and baking.