Scientifically speaking, lactose is a disaccharide sugar derived from galactose and glucose that is found in milk. In simple terms, it is a sugar byproduct of milk. It comes in the form of an off-white powder. On the tongue, it is not nearly as sweet as traditional sugar (sucrose). From a brewing standpoint, the key property of lactose is that it is non-fermentable. Whereas brewing yeasts consume simpler sugars such as glucose to create two essential components of beer (alcohol and carbon dioxide gas for carbonation), they cannot consume lactose. Therefore, whatever amount of lactose you add remains in the beer, allowing you to fully control the level of sweetening without heightening the levels of alcohol or carbonation.