Milk / Sweet Stouts are basically stouts that have a larger amount of residual dextrins and unfermented sugars that give the brew more body and a sweetness that counters the roasted character. Milk Stouts are very similar to Sweet Stouts, but brewers add unfermentable sugars, usually lactose, to the brew kettle to add body and some sweetness.
Average alcohol by volume (abv) range: 4.0-7.0% --BeerAdvocate