You might call kefir the original smoothie.
Like yogurt, kefir is made with bacterial cultures and milk; but unlike its curdled cousin, kefir has a milkshake consistency and can be enjoyed as a drink instead of as a food.
It contains probiotic bacteria similar to those in yogurt, as well as yeasts that give kefir a slightly alcoholic taste. Kefir has not been studied as extensively as yogurt, but because it contains bacteria similar or identical to those found in yogurt, it may have some of the same beneficial effects, including improved lactose digestion and tolerance (Journal of the American Dietary Association, 2003, vol. 103, no. 5).