Food fermentations are perhaps the oldest food preservation method. "Good" microorganisms ferment the sugars and nutrients in a food to produce byproducts that usually preserve the food in some manner. For example, cucumbers are naturally fermented by the bacteria on them to produce lactic acids. These acids "pickle" them and preserve them by inhibiting other spoilage bacteria from growing. Milk is fermented to produce acids that create cheese, yogurt, and other products. In most cases fermentation alone cannot produce a shelf stable (room temperature) food product.
NCHFP Resources - General info, includes Pickles, sauerkraut, and yogurt.