The two most important types of fermentation in your daily life:

• Alcoholic fermentation — yeast converts glucose into ethanol and CO2. This gives us beer, wine, and bread (the alcohol bakes off, the CO2 makes it rise)
• Lactic acid fermentation — bacteria convert glucose into lactic acid. This produces yogurt, sauerkraut, kimchi, and it is the backup your muscles fall back on when oxygen runs short during intense exercise

Both are anaerobic processes, meaning they happen without oxygen. When oxygen is available, cells switch to aerobic respiration instead, because it pulls far more energy out of the same sugar. Fermentation is the backup generator.