Buttermilk
Buttermilk was originally the bi-product of making butter. Modern production uses a specific type of bacteria which is added to nonfat or low-fat milk and creates a thick, tangy milk flavor. 

Liquid Buttermilk
There are two types of commercial buttermilk, cultured and acidified. Cultured buttermilk is made by using a lactose-producing bacterial strain that thickens the milk while the acidified version is made by adding tartaric or citric acid to milk. The acid thicken the milk and produces a sour flavor.

Buttermilk Powder
The buttermilk is first pasteurized, then concentrated with an evaporator and finally dried (spray or roller dried) to produce buttermilk powder. Buttermilk powder is low in fat and provides an excellent source of protein. It is also a source of calcium, phosphorous, riboflavin and vitamin B₁₂. (Milkingredients.ca)

Tips For Storing or Using Left Over Buttermilk

Refrigerate - Buttermilk stores for weeks after the expiration date and works very well for recipes that call for buttermilk.
Freezing - Place buttermilk in 1/2 to 1 cup freezer containers. Thaw in the refrigerator.  The thawed milk is appropriate for cooking but probably not for drinking.
Leftover Buttermilk?  Here are Recipes Featuring Buttermilk
Cups	Title
2	Buttermilk Pound Cake - A slightly reduced fat buttermilk uses low fat buttermilk, and margarine.
1	Cheddar-Buttermilk Cornbread - Conventional cornbread kicked up with cheddar cheese.
2	Buttermilk Panna Cotta This recipe lightens up the usual all-cream version and is served with fresh tropical fruit.
1 1/4	Buttermilk Pumpkin Pie Creates a lighter, less dense pie than the traditional version
2	Buttermilk Waffles - from Martha Stewart
1	Buttermilk Pie - from Emeril
2	Feather light Buttermilk Pancakes -The secret to making these amazing pancakes is the lumpy batter. So whatever you do, do not over mix the batter!
1/2	Buttermilk Dressing - Uses cayenne, sour cream, buttermilk, onion and lime juice