Buttermilk is a fermented dairy product that contrary to its name does not contain butter. Actually buttermilk technically is the liquid that is left over after making cultured butter, and so therefore should be virtually fat free. Today though, most buttermilk is made by culturing fresh milk. Buttermilk has a sour flavor like plain yogurt, but is must more fluid. Its a bit of an aquired taste, but if you don't care for its plain yogurt like flavor, you can make it more palatble by mixing in a good grass-fed flavored yogurt or by blending it up with some fruit. A glass of low/no fat buttermilk is quite healhty and has more probiotic bacteria in it than a container of Activa yogurt, but costs a lot less. If you can find a good grass-fed buttermilk in your area, give it a try.
Below is a fantastic video produced by Joe York for the Southern Foodways Alliance about Earl Cruze, a dairy farmer from Cruze Farm in Knoxville Tennessee, who has an extreme passion for making and drinking buttermilk.