A very dark, heavy, top-fermented beer made from pale malt, roasted unmalted barley, and often caramel malt. Stout was first introduced by Guinness as an extra stout version of their porter. The new stout was darker, hoppier and richer than porter, which it gradually overtook in popularity. A distinction is drawn between sweet stout and dry stout: although both are highly hopped, sweet stout is less bitter than dry stout.
Originally this was an Irish version of Porter. A rich, dry, extra-dark, black opaque ale. A proper dry stout is intensely roasty, with plenty of hop bitterness. It should be top-fermented, with the attendant fruitiness and complexity. Low to medium bodied, the distinguishing feature is the requisite roasted coffee like flavor and aroma from the use of roasted barley which is required but can be at low levels. It starts with a taste of malt and caramel and ends with a dry-roasted, bitter taste. Hop bitterness is medium to high, but malt should still dominate slightly. There may be very low to medium diacetyl. There is just enough English variety hop flavor present to offset the malt. Thus, the hop flavor is barely noticeable and there is no hop aroma.
The origins of stout are even more obscure than those of porter, of which stout is probably an offshoot. Although stout is mentioned as early as the late 1600s, most likely it was a strong dark ale of the type now called "old." For centuries, the British used dark malts to balance the sweetness of the old-style unhopped ales and continued to use them after hops were generally adopted. These were brown malts, used for all or a large proportion of the grist. Black malts were first introduced in the 1830s in the London porter industry, and chocolate malt and roasted barley followed later. Guinness, like many great brewers, first gained fame with porter. Stout seems to have really come into its own as porter entered its long decline.