Russian Imperial, dry, sweet, milk, American, oatmeal: whichever it is, it's the beautiful stout. A direct descendant of the porter, it got its moniker because it was a stronger, roastier version of its mother beer--a "stout" porter, if you will. Over the years the "porter" was dropped from the name and various transformations took place in accordance with the local tastes and availability of ingredients leaving us with a wide array of this beautiful dark beer.
The dry Irish version, thanks to the likes of Guinness, is the most famous and many breweries around the world brew their own interpretation. The English stout from a brewery like St. Austell tends to be more sweet, especially in terms of the milk stout which has a rounder, sweeter quality. Conversely, the American type is considerably more bold with a lot more hop and roast character; Tiny Rebel's got a good example. Yet, the granddaddy of them all is the Russian Imperial Stout--De Molen is famous for their Rasputin and Northern Monk has got a great one to try--big, brazen, and massively intense; bring a friend if you intend to overindulge!