it's basically the mercantile class: the growth of importers, tradesmen, manufacturers, shopkeepers etc. Previously society had consisted chiefly of a few aristocrats and clergy and a lot of peasants, with a comparatively small middle class. The peasant class has practically vanished now, to be replaced after the industrial revolution by the working class, but they have also shrunk a lot since the 19th century. These changes have been gradual and you could probably date them to the time of the Black Death, when so many people were killed that farm and town workers were greatly in demand and able to name their price.