How did Karl Marx describe class conflict?
Karl Marx was a 19th century philosopher whose works primarily criticized socioeconomic inequality in Europe and describe the means by which this might persist or be remedied. We know his body of work as Marxism, and any discourse which analyzes or challenges class structures may be considered Marxist.
Marx was highly critical of socioeconomic systems where much of the wealth is concentrated among a small portion of society, as it is in capitalist and (historically) feudal societies. Marx described the conflict between the upper class bourgeoisie who own the resources and labor in a system and the working class proletariat who must labor in order to survive. Capitalism can create the illusion that because an individual is working, they are in control of their actions. In reality, the raw materials, machines, and everything the worker needs to create their product is owned by the upper class. The worker may earn enough to survive, but all surplus produced belongs to and benefits the upper class.
The conflict at work here may be the proletariat's resentment of the bourgeoisie or something far more dramatic like a revolution. Marx's work inspired the Communist Revolutions of the 20th century. Marx believed the only way to remedy the parasitic relationship between the bourgeoisie and proletariat was to instigate a revolution and overthrow the ruling class. Such revolutions were the case in Russia, China, and Cuba in the early 20th Century. Marx also believed an ideal follow-up was to establish a truly communist society where all people work and benefit equally, but this was not always the case after a revolution.