The American Homebrewers Association recognizes five distinct substyles. Classic Irish-Style Dry Stout is best known under the Guinness label. This style is very bitter, with a dry-roasted finish. Stout has fuller body and less roasted character. It is common to add lactose before bottling to enhance this sweetness.
Oatmeal stouts are smooth and mild, with oats making up an important part of the grist.
The most commonly found example of an oatmeal stout is Samuel Smith's Oatmeal Stout. Finally, we have the Imperial Stout, possibly so called because they were made to be shipped to Russia. There is still a stout brewing tradition in the Baltics, possibly created in competition to the British. Color should be dark copper to black. Maltiness is rich, with very little diacetyl.