Somewhat related to yogurt, Kefir is a fermented milk drink that originated in the Caucasus region. It is prepared by inoculating cow, goat, or sheep's milk with kefir grains. Traditional kefir was made in sheepskin bags that were hung near a doorway; the bag would be knocked by anyone passing through the doorway to help keep the milk and kefir grains well mixed.
Kefir is delicious and purportedly confers many health benefit, read the WIKI article. Is kefir for you? Find out by buying a quart, available at Trader Joe's, Whole Foods, maybe even your upscale local super market. It usually comes plain and strawberry flavored. If you like it from the start or develop a taste, you may well get tired of paying $3.00 and up per container, in which case it's time to get started. Note that a sheepskin bag is not required, widemouth glass jars will do just fine.
Read the common method for making kefir INSTRUCTIONS. Then contact Wendy Dutsky, lunarlander [at] yahoo [dot] com, ask her if she has kefir grains available. If yes, she'll ask you to send her $3 cash to cover mailing expense. And soon you'll be in the kefir business.