That's a pretty broad question, but I would first start off by looking at Marxism and exploring Marx's views on the Proletariat. The industrial revolution allowed him to focus largely on the means of production within factories, and he says a lot about the social classes that seperated the Bourgeoisie and Proletariat. If you can find examples of working class jobs now and use Marxist theories to analyse how they are being exploited by the upper class then that should hopefully give you a starting point. How you define them is mostly up to you as social class is rather fluid, but Marxism would be the most obvious starting point for exploring social class.