Buttermilk is the white liquid remaining around the butter curds once cream has been churned into butter. This was the original milk with all the fat removed. Today, buttermilk is often commercially produced by adding bacterial culture to milk. 

Nutritional value
An excellent source of calcium, with a high protein content and only 1% butterfat. 

Uses
It's a natural emulsifier, excellent for making cold sauces with herbs and lemon juice.

In baked goods, buttermilk produces a tenderer dough or pastry and enhances flavor. It gives a nice texture to products like muffins, bread, biscuits and pancakes. 

It adds a wonderful flavor to salad dressings, dips and marinades. 

Cooking
It can be used on its own or heated, which will make it creamier. It's popular in Northern France sweetened with brown sugar. 

Substitute
Replace buttermilk with fresh milk (250 ml / 1 cup) to which you've added 10 ml / 2 tsp. white vinegar.