Tradition: The name "Porter" was first used in 18th century England to describe a dark brown beer made with roasted malts. Because of their popularity, brewers made them in a variety of strengths. Stronger, higher ABV beers were later called "Stout Porters". The term "Stout" has since become firmly associated with dark beer, rather than just strong beer.
Taste: Today, Belgian Stouts are more common than Belgian Porters and subdivide into sweeter and drier, and stronger and weaker versions. Sweeter Belgian Stouts resemble the almost-defunct British style "Milk Stout", while the stronger ones are sometimes described as Export or Imperial Stouts.