I'm a big fan of just about every dairy product out there. I'm also a big fan of edible kitchen science experiments. Knowing that, my friends J and K offered me some kefir grains so that I could make my own kefir.
I was mostly ignorant of kefir at that point. It's a cultured milk product similar in taste to yogurt or buttermilk. It differs in that the bacteria and yeast that help culture the milk live in a matrix of protein, lipids, and sugars. These are known as kefir grains. The grains, which look like little pieces of cauliflower, grow over time and sometimes split off new colonies. J and K's colony had grown and produced a new bud. They brought it over on a cup of milk, and also brought me a cup of the final product.
To make kefir, you put the buds in some fresh milk and let it sit on the counter for a day or two. I usually put it in a small bowl and cover it lightly with plastic wrap. The bacteria and yeast need some oxygen so it's a good idea not to cover it too tightly. When I remember, I also swirl it around once or twice to help the colony get fresh nutrients. The milk thickens and turns into kefir. Once it's done you can store it in the fridge. When you want to make more, fish out the buds and put them in fresh milk. My bud was about the size of a grape when I got it two months ago. Since then it's doubled in size and split off two grape-sized buds.
The kefir tastes a lot like buttermilk and can be used in recipes calling for buttermilk or yogurt. I eat it on cereal or in smoothies. I like it because not only is it cheap, it's also a good way to finish those gallon containers of milk before they go bad.
Nutritionally, it's similar to milk but has a lot of helpful probiotics. Since the bacteria digest the lactose, it's supposedly more easily digested by people who are lactose intolerant.
Here's the wikipedia link if you're interested in learning more about kefir