Available by the ounce or the pound.
Lactose is a milk sugar that is only about 1/4th as sweet as other sugars (sucrose, dextrose). Most importantly it won't be fermented out by beer yeast, which means your beer will remain sweet and won't be converted to ethanol. It can be added to the boil (in the last 5 minutes) when brewing beer.
Traditionally used in beers like milk stouts, it's become a common adjunct in Milkshake IPAs (sweet, fruity IPA style beers).