The Buttermilk you buy in the store is a cultured milk. Often used for baking for it’s flavor and the acidity which reacts to baking soda and baking powder.
Typically you can substitute Buttermilk for sour milk OR sour milk for Buttermilk. Need sour milk for a recipe? Read this. How to make sour milk.
My purpose on this page is to give a little information on how and why it might be used in a recipe and what can be substituted for it. Want to know more? Wikipedia has some good information and history if you are interested. Wikipedia – Buttermilk