The term stratification in sociology is usually applied to studies of structured social inequality; that is, studies of any systematic inequalities between groups of people, which arise as the unintended consequence of social processes and relationships. When we ask why there is poverty; why Black people or women in the United States are disadvantaged vis-à-vis (respectively) Whites and men; or what chances someone born into the working class has of achieving a middle-class position; we are posing questions about social stratification.
Social stratification is thus at the heart of macrosociology—the study of whole societies, in comparative perspective, in an attempt to understand processes of social stability and change. Social stratification begins from Weber's limiting cases of the more traditional status-based society (for example, societies based on ascriptive categories such as estates and castes, or where there is slavery so that inequalities are legally sanctioned), and the polarized but more fluid class-based society (typical of the modern West), where there is a greater element of achievement, where economic differences are paramount, and inequality is more impersonal. Status formation and class formation thus represent the two extreme poles of social integration—the ways people in a society relate one to another.
Studies of social stratification have three objectives. The first is to establish the extent to which class or status systems predominate at the societal level such that they are constitutive of modes of social action. Hence, to make the claim that Britain is a class society one would need to show that class relationships underlie predominant modes of social action, and represent the fundamentals of social integration. The second is the analysis of class and status structures and the determinants of class and status formation: for example, to pose questions such as why there is no socialism in the United States, or why the British working class did not produce a communist revolution, is to pose questions about the degree of class formation in society. Many sociological and historical studies have attempted to explain variability in the degree of such class formation. Finally, social stratification documents inequalities of condition, opportunities and outcome, and the ways in which groups maintain class or status boundaries. In other words, it addresses the question of social closure, and investigates those exclusionary strategies by which groups maintain their privileges and other groups seek to gain access to them. Often class and status interact in interesting ways. For example, advantaged classes may attempt to develop the characteristics of status groups in order to routinize, justify, and thereby maintain their privileges: the nouveaux riches the world over are notorious proponents of this strategy. Similarly, the complex articulation of class, race, age, and gender differences have come increasingly to interest researchers investigating the multifarious processes of social stratification in modern societies: the work of Joan Huber is a good illustration of this development (see, for example, Sex Stratification, 1983).
At the most general level, therefore, social stratification is concerned in different ways with the issues of class and status-group formation as the key to understanding social integration; that is, the extent to which social relationships are cohesive or divisive, and the consequences of this for social order. Useful overviews are John Scott, Stratification and Power (1996) and Rosemary Crompton, Class and Stratification: An Introduction to Current Debates (2nd edn., 1998).