What? Porters and Stouts are heavily hopped, dark ales that are top fermented (if they were bottom fermented, they would be dark lagers, and they would be fizzy).
Where? You find most brewers around the world will attempt a stout or a porter. They have quite a bit of potential in terms of barrel ageing, added spices, and interesting malts. Not only that, many beer fans rate dark beer highly. So not only do brewers like the challenge, but if they make a really good one, it is a style that's appreciated.
When? Stouts and Porters are really part of the beginning of commercial brewing in Britain in the 18th and 19th century. Porters (the name is said to come from the workers who drank it) were made up of brown malt and became popular because of their heavily hopped and heavily malty style. Stout is said to mean strong, and thus beers that were stronger would be called "stout porters" and after a while, just "stout".
How? As they are of a heavier style, the joy with a Stout/Porter is they are beers that take time to drink. Pour a glass, preferably when you are in front of the fire and savour.