Marx, Weber, and Durkheim all highlighted the significance of class differences and sought to understand both their causes and consequences. Du Bois and Addams worked to ameliorate the excesses of social stratification. Social class is a touchy subject in the United States. The term social inequality describes a condition in which members of a society have different amounts of wealth, prestige, or power. Sociologists refer to social inequality that is built into the structure of society as stratification. This is the structured ranking of entire groups of people that perpetuates unequal economic rewards and power in a society. To help understand stratification systems, recall the distinction between ascribed and achieved statuses. An ascribed status is a social position assigned to a person without regard for that person's unique characteristics or talents. Race, gender, and ethnicity are examples of ascribed statuses. An achieved status is a social position attained by a person largely through his or her own efforts, such as becoming a corporate executive or graduating from college.

The most extreme form of legalized social inequality is slavery. Castes are hereditary ranks, usually religiously dictated, that tend to be fixed and immobile. The estate system, also known as feudalism, was a stratification system in which peasants were required to work the land of a noble in exchange for military protection and other services. A class system is a social ranking based primarily on economic position. Daniel Rossides suggests that only 1 to 2 percent of the people in the United States are in the upper class, whereas the lower class consists of approximately 20 to 25 percent of the population. The lower class is disproportionately comprised of Blacks, Hispanics, single mothers, and people with low-paying jobs.

Social mobility is the degree to which one can change the social stratum into which one is born. Stories of upward social mobility are metaphors for the seeming permeability of modern class boundaries. In an open system, the position of each person is influenced by his or her achieved status. In a closed system, there is little or no possibility of individual social mobility. Horizontal mobility is the movement of an individual from one social position to another of the same rank. Vertical mobility is the movement of an individual from one social position to another of a different rank. It can be both upward and downward. Intergenerational mobility involves changes in the social position of children relative to their parents. Intragenerational mobility involves changes in social position within a person's adult life.

In Karl Marx's view, social relations during any period of history depend on who controls the primary mode of economic production. He examined social relations within capitalism, and focused on the two classes that emerged as the estate system declined; the bourgeoisie and the proletariat. The bourgeoisie is the capitalist class that owns the means of production, and the proletariat is comprised of working-class people who are exploited by the capitalist class. According to Marx, this exploitation would inevitably lead to the destruction of the capitalist system because the workers would revolt. They would develop class consciousness and overthrow capitalism in favor of a more equitable distribution system. To do this, though, workers would have to overcome false consciousness engendered by the dominant ideology.

Unlike Marx, Max Weber insisted that class does not totally define a person's position within the stratification system. Weber identified three components of stratification— class, status, and party—that work together to shape individual and group power.

Pierre Bourdieu introduced the concept of cultural capital. For Bourdieu, because culture is hierarchically valued, it is a form of power. The cultural capital of the elite is tied to their control over economic and social resources, and can be used as a form of exclusion from jobs, organizations, and opportunities.

Our cultural preferences are shaped by our social class positions. Occupational prestige and esteem are other ways to describe relative social class positions. Socioeconomic status is a measure of social class that is based on income, education, occupation, and related variables. Income and wealth are material ways to measure social class. Income inequality is a basic characteristic of a class system. By all measures, income in the United States is unevenly distributed. The income gap between the richest and poorest groups in the United States is widening. Wealth in the U.S. is much more unevenly distributed than income. The American middle class is shrinking, and being slowly replaced by two growing groups of rich and poor. Approximately one out of every five households in the United States has trouble meeting basic needs. Absolute poverty refers to a minimum level of subsistence that no family should be expected to live below. Relative poverty is a floating standard of deprivation by which people at the bottom of a society are judged to be disadvantaged in comparison with the nation as a whole. Some sociologists have used the term underclass to describe the long-term poor who lack training and skills. African Americans and Latinos are more likely than Whites to be persistently poor.

The belief in upward social mobility is an important value in the United States. Occupational mobility has been common among White males. Education, gender, and race are important factors in one's upward mobility.

Max Weber saw class as being closely related to people's life chances. Occupying a higher position in a society improves individuals' life chances and brings greater access to social rewards.