In short, lactose is milk sugar. Plain and simple. Just like fructose is fruit sugar.
Lactose occurs naturally in dairy products like milk, cheese, and yogurt. Milk has 12 grams of lactose per serving, while hard cheeses like cheddar cheese only contain 1 gram. Yogurt can have anywhere from 8-14 grams depending on type and fat content.
UDA is able to pull lactose out of milk as a by-product which is then used in many types of packaged foods and beverages. For example, Arizona Wilderness Brewing uses our lactose in their brewing process.
It can also be found as a preservative in bread, waffles, cereals, and breakfast mixes.