Food fermentation is perhaps the oldest food preservation method. "Good" microorganisms ferment the sugars and nutrients in a food to produce byproducts (acids) that usually preserve the food in some manner. For example, cucumbers are fermented by the bacteria on them to produce lactic acids. These acids "pickle" the cucumbers and preserve them by inhibiting other spoilage bacteria from growing. Milk is fermented to produce acids that create cheese, yogurt, and other products. As the acid accumulates it will coagulate or clot the milk. In most cases fermentation alone cannot produce a shelf stable (room temperature) food product. Nearly all fermented foods require canning to preserve or refrigeration.
Dr Nummer's publication on fermenting yogurt can be found here.
NCHFP Resources - General info, includes Pickles, sauerkraut, and yogurt.