Kefir is a tangy, thin, slightly fizzy fermented-milk drink cultured with kefir grains — soft, gelatinous, cauliflower-like clusters that hold a whole community of bacteria and yeasts living together. That mixed culture makes kefir more microbially diverse than yogurt, gives it a clean lactic sourness, a faint yeasty lift and a trace of natural fizz and alcohol, and lets it be drunk by the glass rather than spooned.