Kvass is a sour rye-flavored soft drink. There are two types of kvass: mass-produced kvass, and traditional, homemade kvass.
What is typically sold in supermarkets as kvass is a pale imitation of the homemade product, and is usually simply carbonated water, barley/rye malt syrup, sugar, and caramel coloring. True, real, living kvass is meant to be consumed fresh and is generally too perishable to mass-produce.
Traditional kvass is made by steeping stale rye bread in hot water overnight, typically with a little sugar or grape must. The wort is then pitched with a very small amount of yeast, which, combined with lactobacillus from the air and the maltose and sucrose in the wort, creates a live and active culture. A byproduct of this is a small amount of alcohol (less than 2%), but kvass is not considered an alcoholic beverage in countries where it is commonly consumed.
After four to six hours of fermentation at room temperature, the wort is racked and bottled or kegged. The drink is sweetest a day or two after racking and gradually becomes more sour and earthy with time; after a week it is too spoiled to drink.
Additives (raisins, whole or puréed fruit, spices) are very common.