Before going into the history and description of Stouts, one must first give props to its predecessor, the Porter.
Porters, a dark ale favored among London's working classes, was first developed in the early 1700s. Street and river porters provided an eager market for this new, energizing beer. The word "stout", after the fourteenth century, had taken on as one of its meanings "strong", and was used as such to describe strong beers, such as the Porter. "Stout" as in stout porter, was the strong, dark brew London's brewers developed and the dark beer that gave us what we think of today as the typical stout style.
The first stouts were produced in the 1730s. The Russian Imperial Stout was inspired by brewers back in the 1800's to win over the Russian Czar. "Imperial porter" came before "imperial stout" and the earliest noted use of "Imperial" to describe a beer comes from the Caledonian Mercury of February 1821, when a coffeehouse in Edinburgh was advertising "Edinburgh Ales, London Double Brown Stout and Imperial Porter, well worth the attention of Families".
Guinness had been brewing porters since about 1780 and are famous for their Dry or Irish Stout. Oatmeal stout beer is one of the more sweeter and smoother of the stouts. And for proof that we live in an evolving society, there's Oyster Stout and Chocolate Stout. The first known use of oysters as part of the brewing process of stout was in 1929 in New Zealand.
Originally, stout meant "proud" or "brave", but morphed into the connotation of "strong" after the 14th century. Why on earth should this brave and strong beer style not have its own day of celebration?