Kefir is kind of like yogurt, only more liquidy and (IMHO more delicious). It has like 40 types of beasties in it versus the scant handful in yogurt. You get these cultures commonly called "grains" (but they're not really grains). And you drop them in your culture jar along with your liquid (most use milk, I use coconut milk). You cover it losely with something that can breathe (i use a coffee filter with a rubber band) and put it in a cupboard and let it ferment for a couple of days.

It gets sour and lovely in 12-48 hours or so. The bacteria turn lactose into lactic acid which preserves the milk, something in there is making a bit of alcohol. Then you strain out the "grains" and put a lid LOOSELY on the jar (because it gives off a bit of C02 and you don't want it to explode). You start a new batch with the grains, or you can rest them. And it is very slightly alcoholic and kind of fizzy and wonderful.

I got a buzz of Kambucha tea (similar thing, but made with tea) but never have off of kefir. I'm a true alcohol light weight.