Kefir is milk fermented by combination of bacteria and yeast. Kefir is easy to make at home if you have the starter.
Unlike yougurt, it cannot be started by the remnants of the previous batch: you will get a pleasant generic cultured milk, prostokvasha, if you try. Kefir starter, also called kefir grain looks like small (1/2" to 3" long) off-white perly irregular clouds there yeast live. It has to be added to the milk to ferment it, and strained out before drinking. Kefir starter is perfectly edible, with texture similar to very dense cheese curd, but it grows slowly. You don't want to waste it.
Kefir is best if made with whole or 2% milk. Fat free milk yields kefit with too flat and acid taste. Kefir has roughtly the same calorie count as fresh milk, but some of the energy is consumed by starter.
1/2 cup of the starter turns a quart of milk in kefir overnight, at room temperature. Due to yeast fermentation, kefir contains some gas. Use a container with enough room for it. Mature kefir looks cloudy or curdly, with some whey separation. Starter tends to float and cultivation starts in the top half of the milk. It helps to stir or shake milk once it starts to separate. Once the milk throughly cultured, strain through the fine wire mesh strainer. Kefir whey goes down the strainer easily, but the curd is thick and clings to the grain. It helps to stir kefir with a spoon against the strainer to release the grain. Sometimes I catch part of the why in a cup and re-use it to rinse the thick part off the grain. Stir the strained kefir a little. Excessive agitation will release the gas. Kefir is best refrigerated. Unlike homogenized and artificially thickened kefir from the store, homemade kefir is rather runny with occasional "knots", or small seeds. If you don't like this knotty texture, strainyour kefir again through the folded cheese cloth. I prefer not to overhandle kefir and keep it knotty and sparkly.
Kefir is good straight from the refrigerator, or half-and-half with sparkling water. It can be used in pancakes instead of milk. Also, one part of kefir to two parts of fresh milk makes great homemade ricotta. The proportion can vary depending on your taste and contents of your refrigerator. Kefir strained through tightly woven cotton (not cheesecloth!) makes soft spreadable cheese similar to creamcheese, but much lighter. Remaining whey is good for pankeces, and as a conditioner for your skin and hair (rinse throughly, it smells). Kefir keeps in a refrigerator at least for a week, getting more sour, but perfectly drinkable and healthy. Live kefir is considered healthy food; its secret was teased out of Caucasian Mountains tribes along with the notion of it's almost magical virtue. I believe it is beneficial in the same way the yogurt is. The cultivation in kefir is more complex and probably yields even wider variety of beneficial micronutrients.
Put the grain back into you cultivation jar and add milk, or, if you want to give your grain a rest, add water to it (a cup of water for 1/2 cup of grain is enough), and put in refrigerator. The grain will keep refrigerated in water at least for two weeks. If I'm planning to leave it for more than three days, I add some sugar to the water to make sure the grain will not starve.