The key component to examine with regard to stratification according to Karl Marx is the economic system. Marx believed that capitalism was a system that caused inequality and oppression, which in turn would cause conflict between different groups. Some of these groups could be capitalists and others workers, but the most important categories of people that Marx talked about were bourgeoisie (the ruling class) and proletariat (the working class). Marx felt that both groups were oppressed by capitalism, but he also believed that the proletariat held the power to overthrow capitalism through revolution.
According to Marx, capitalism is an economic system based on private ownership of property and a market economy in which goods, services and capital are traded for money. The bourgeoisie own the means of production and their labor power is sold for wages. Proletarians sell their labor power for wages and must work for a living. The proletariat is exploited by the bourgeoisie because it does not receive all of its produce as wages. This leaves them with no other option than to sell their labor power again in order to survive.
Marx believed that class structure was determined by the economic system at any given time period. In other words, if there is an economic system where people own things and trade them then those people will be considered upper class or bourgeoisie; whereas if there is an economic system where people do not own anything but are paid wages then those people will be considered lower class or proletariat.