The concept of social class…does not refer to income levels, educational attainments, or other static measures of social stratification. A social class in this sense is a social force acting in relationship to other social forces. In the case of the working class, it acts primarily in relationship to the capitalist class which employs the active workers in its ranks. Indeed, the working class is defined by its relationship to, dependence on, and opposition to the capitalist class. This relationship begins in the workplace at “the point of production,” but it extends throughout society, influencing politics, culture, and the quality of life in general.
[T]he working class cannot be understood simply by the products it makes or services it performs. Rather, it is a complex, constantly evolving social formation. In objective terms, its existence as a class is defined by its relationship to capital, not by occupation or industrial status. Moreover, as a social class, it includes not only its jobholders, but those who grow up in its homes and those who retire from the labor force to grow old; in its ranks are those who work at home and raise its children, and those whom it hires to run its unions. It is a web of individuals, families, organizations, and communities, some of which are hostile to each other at one level or another, but who are tied together by a common relationship to an alien economic/social force — capital. Far from disappearing, as some argue, the working class has grown even as it has changed.
–Kim Moody, An Injury To All: The Decline of American Unionism (1988, Verso)