A cousin of the original eau-de-cologne, which term has since become generic. The name is derived from the street number of the house of the original manufacturer.
In Irish English it's read as "four-seven-eleven"; according to Wikipedia, it's pronounced "fortyseven-eleven" or "Siebenundvierzigelf". Wikipedia also tells us that:
The German Navy during World War II issued vast amounts of 4711 perfume to the submariners of the U-Boat fleet. As there were limited facilities and few opportunities for bathing, the perfume would be used in an attempt to improve the odour aboard the vessel. Crews typically didn't use much of the perfume and would take home bottles as presents for mothers, wives, or girlfriends.