The relative importance and definition of membership in a particular class differs greatly over time and between societies, particularly in societies having a legal differentiation of groups of people by birth or occupation.
In the well-known example of socioeconomic class, many scholars view societies as stratifying into a hierarchical system based on economic status, wealth, or income.
Using wealth as a dimension, many have used a bi-partite model to view societies, from ancient history to the present day:
Karl Marx famously claimed that the primary social division was between a "ruling class" and a labouring class. Under slavery, this division corresponds to that between the slave-owners and the slaves, while under feudalism, it corresponds to that between lords and serfs. Under capitalism, the capitalists (the bourgeoisie) exploit the working class (the proletariat, or in other words the wage-earners). See labor theory of value.
Note that the Marxist definition of social class is based on how money is earned, not how much money is earned. The bourgeois are those who own the means of production (i.e. business owners) and hire other people to work for them; the proletarians are those who do not own means of production, and earn their living by working for the bourgeois and receiving wages. Therefore, there is no "middle class" per se - only poorer-than-average bourgeois and richer-than-average proletarians.
Power and stratification
Many sociologists and historians see "higher" classes as controlling subordinate classes.
Some view this as an oligarchy, i.e., a ruling class with the assistance of a middling class dominating the working class. Other theories are less critical of the upper classes, seeing them as protectors or innovators (not mere leeches).
Ideas of Max Weber
When sociologists speak of "class" they usually mean economically based classes in modern or near pre-modern society. Modern usage of the word "class" outside of Marxism generally considers only the relative wealth of individuals or social groups, and not the ownership of the means of production.
All three dimensions have consequences for what Weber called "life chances".
Ideas of Karl Marx
In Marxist terms a class is a group of people with a specific relationship to the means of production. Marxists explain history in terms of a war of classes between those who control production and those who actually produce the goods or services in society (and also developments in technology and the like). In the Marxist view of capitalism, this is a conflict between capitalists (bourgeoisie) and wage-workers (proletariat). For Marxists, classes are antagonistically opposed to one another. This antagonism is rooted in the situation that control over social production necessarily entails control over the class which produces goods -- in capitalism this is the exploitation of workers by the bourgeoisie.
The most important transformation of society for Marxists has been the massive and rapid growth of the proletariat in the last two hundred and fifty years. Starting with agricultural and domestic textile labourers in England, more and more occupations only provide a living through wages or salaries. Private enterprise or self-employment in a variety of occupations is no longer as viable as it once was, and so many people who once controlled their own labour-time are converted into proletarians. Today groups which in the past subsisted on stipends or private wealth -- like doctors, academics or lawyers -- are now increasingly working as wage labourers. Marxists call this process "proletarianisation," and point to it as the major factor in the proletariat being the largest class in current societies in the rich countries of the "first world."
The increasing dissolution of the peasant-lord relationship, initially in the commercially active and industrialising countries, and then in the unindustrialised countries as well, has virtually eliminated the class of peasants. Poor rural labourers still exist, but their current relationship with production is predominantly as landless wage labourers or rural proletarians. The destruction of the peasantry, and its conversion into a rural proletariat, is largely a result of the general proletarianisation of all work. This process is today largely complete, although it was arguably incomplete in the 1960s and 1970s.
Dialectics, or historical materialism, in Marxist Class
Marx saw class categories as defined by continuing historical processes. Classes, in Marxism, are not static entities, but are regenerated daily through the productive process. Marxism views classes as human social relationships which change over time, with historical commonality created through shared productive processes. A 17th-century farm labourer who worked for day wages shares a similar relationship to production as an average office worker of the 21st century. In this example it is the shared structure of wage labour that makes both of these individuals "working class."
Objective and subjective factors in class in Marxism
Marxism has a rather heavily defined dialectic between objective factors (i.e., material conditions, the social structure) and subjective factors (i.e. the conscious organization of class members). While most Marxism analyses people's class status based on objective factors (class structure), major Marxist trends have made excellent use of subjective factors in understanding the history of the working class. E.P. Thompson's Making of the English Working Class is a definitive example of this "subjective" Marxist trend. Thompson analyses the English working class as a group of people with shared material conditions coming to a positive self-consciousness of their social position. This feature of social class is commonly termed class consciousness in Marxism. It is seen as the process of a "class in itself" moving in the direction of a "class for itself," a collective agent that changes history rather than simply being a victim of the historical process.
Non-economic conceptions of class
In contrast to simple income--property hierarchies, and to structural class schemes like Weber's or Marx's, there are theories of class based on other distinctions, such as culture or educational attainment.
For example, Bourdieu seems to have a notion of high and low classes comparable to that of Marxism, insofar as their conditions are defined by different habitus, which is in turn defined by different objectively classifiable conditions of existence. In fact, one of the principal distinction Bourdieu makes is a distinction between bourgeois taste and the working class taste.
Class in different parts of the world
At various times the division of society into classes and estates has had various levels of support in law. At one extreme we find old Indian castes, which one could neither enter after birth, nor leave (though this applied only in relatively recent history). Feudal Europe had estates clearly separated by law and custom. On the other extreme there exist classes in modern Western societies which appear very fluid and have little support in law.
The extent to which classes are important differs also in western societies, though in most societies class as an objective measure has very strong empirical effects on life chances (e.g. educational achievement, life-time earnings, health outcomes). Only in the strongly social-democratic societies such as Sweden is there much long-term evidence of the weakening of the consequences of social class.
The effect of class on vote or life-style is more variable across countries and over time.