Kefir is a fermented milk drink that originated in the Caucasus region. It is made by adding kefir grains to milk and allowing it to ferment. Kefir grains are a symbiotic culture of yeast and bacteria that look like small cauliflower florets. The fermentation process takes about 24 hours and results in a slightly tart, carbonated beverage with a consistency similar to yogurt.
While store-bought kefir is usually pasteurized, you can also find unpasteurized versions at some health food stores. Pasteurization kills harmful bacteria, but it also destroys some of the beneficial probiotic bacteria found in kefir. These probiotics are what give kefir its unique health benefits, so if you're looking for the most nutritional value, choose unpasteurized kefir over pasteurized.