Guinness is a stout or 'Porter'.
The style was originally created in London by accident. Roasting Barley for ale was common but when it was over done (or burnt
) it turned into a darker stronger more bitter tasting brew. It did not really catch on middle class London but it became the brew of the working class porters of which there were many as it was sold as a cheap beer and soon became know as a pint of porter.
Arthur Guinness saw what others did not that the working classes had developed a great love for this dark beer and drank a lot of it
and the rest they say is history