Karl Marx contributed his part economic / part sociological theory, called Marxism, to the field of sociology. According to Marxism, it was unbalanced and unfair economic system that created tensions within society and caused class conflict. He argued that the wealthy and those who owned businesses exploited the working class in order to gain more wealth. It was because of this exploitation, that the working class becomes obedient while still feeling the tension of being inferior to he wealthy.
From a sociological standpoint, he argued that societal roles and class conflict played a large part in human behavior and how societies functioned. Modern discussion of sociological theory often references Marx's works when discussing class and conflict. He is considered one of the founding fathers of sociology and his work is still highly referenced today. It is mostly used to examine if and how people and society as a whole behave and respond in reaction to possible oppression from and conflict with higher classes of society.