This is where it gets interesting.

There are varieties of beer. And oddly none are really considered beer. Beer is really a generic name that includes all varieties. When one is getting down to brass tacks, there sincerely is no particular drink called beer. Well, there are things like apple beer and birch beer, but, that is a different topic completely.

There are ales. Ales are fermented at high temperatures and a fruity aroma are a common characteristic.

Ales are more numberous than any kind of beer. They have a range of color from golden to black. The other color designations are amber and brown. They have a range of flavor based in sweetness. The flavors are from sweet to bitter. 

The very darkest ales are considered stout. Guinness Stout is black and bitter. 

Mackeson Stout is black and sweet. Why is it sweet? Because it uses milk for its sugar. Lactose. Now, milk stouts use a special species of yeast called, Saccharomyces cerevisiae. Milk Stout was originally begun in the late 1800s with whey left over from making cheese. It was actually a by-product of cheese making once found that whey fermented well into a beverage with a kick. 

Mackerson Stout became a beverage in 1907. AB InBev now owns the beer.