Kefir is a fermented milk drink made from what is known as grains, with the actual grains themselves made of lactic-acid forming bacteria and yeasts in a matrix of fats, proteins and sugars. They are not actually a grain, but are called that for lack of a better term. They make a construction that looks like a piece of soggy white popcorn.
The essential ingredient, kefiran, is a bit ropey in texture and can be yellow or white and about the size of walnuts. There are many constituents to the kefir including sometimes alcohol from the fermentation process and vitamins and minerals.
Vitamins: A, B1, B2, B6, B12, D, K2, folic acid, nicotinic acid
Minerals: calcium, iron, iodine
The lactose in the milk gets broken down to feed the bacteria, which convert it to lactic acid. Other bacteria, the same present in Swiss cheese, break some of the lactic acid down into propionic acid. Later on in the ferment, lactose is broken down into ethanol and carbon dioxide, giving the kefir a bubbly, slightly fizzy zest.
Lactose intolerance and dairy allergies
Those with lactose intolerances are usually able to tolerate kefir, because there is very little left after the milk is fermented. Fermented products are known to take longer to digest also, removing the dire need to completely remove lactose from the diet.
You can also do non-dairy ferments with milk kefir if you want to.
How do you make kefir grains?
Kefir grains cannot be made from scratch, but can be frozen or dried. Nobody knows where it came from, but the word is used in Russian, Polish and English.