|A nice Imperial Stout in my favorite glass|
Other than being dark, stouts have a large range of tastes, textures and alcohol levels by volume (ABV). Porters started out as rather strong beers with about 8% abv. Over time, they became cheaper and weaker and evolved into the Gatorade of the day, sweetish, and refreshing. Aging usually adds flavor but takes time and money. Porter brewers discovered that a small amount of aged beer could be used to flavor new beer and make it good enough to sell. Irish stout dispensed with even more ingredients (except the cheap black color of burned barley) and made a thin but refreshing drink of moderate abv. Today, Guinness (~ 4% abv) is the modern version. When fresh, it's quite a nice summer beer. However, it sours very easily, and if not used quickly becomes quite unpalatable - if you have a palate... Beware the "Irish" pub serving stale or insipid Guinness. You are better off with the bottled version.
The acme of the porter/stout genre is the Russian Imperial Stout or Imperial Porter. Originally, Imperial stout was brewed in London for the Empress Catherine the Great of Russia. It was a strong and flavorful concoction (~10% abv). When done right, imperial stout is a sublime example of the brewer's art. My thanks to big Jeff at Redbones for a really fine bottle.
In this context, I'll paraphrase Benjamin Franklin, "Imperial Stout is proof that the almighty wants us to be happy".