Lactose, or milk sugar, is a non-fermentable sugar that is used to add sweetness to drier beers, usually stouts.
Usage: See your recipe. Its normal usage is between 8 oz and 1 lb per 5 gallons of beer.
WARNING! Do not use lactose if you are lactose intolerant, and do not let friends who are lactose intolerant drink a beer made with lactose.