Commercial "kefir" in the U.S. is not made from kefir grains and will not generate kefir grains, though it is a cultured milk product and can be used to culture milk, but it will never generate grains. Kefir grains can be obtained commercially or from trading posts listed on my website
http://wildfermentation.com (under fermentation resources). Kefir is similar to yogurt, but it is a distinctive microbial community and different from yogurt. Where yogurt can be solid, kefir is always a liquid (unless strained), and because it has yeast it can be slightly alcoholic and bubbly. Yum!