Marx's theory of social class is founded on the idea of conflict of interest defined by the property system. Marx puts the point this way in the Communist Manifesto: “History is a history of class conflict.” And his inference from this fact: “Workers of the world, unite; you have nothing to lose but your chains” (Marx and Engels 1848). Individuals belong to classes depending on their position within the social property system. The social property system defines the access and use enjoyed by different groups of the resources available to a society at a given period of history. The primary resources are capital, land, and labor. (We might now want to add "knowledge" and "data" to this list of categorical resources.) Individuals belong to classes defined by the type of access and use they have to what kinds of resources.
This is a structural definition of the concept of class. A person’s class is defined by his or her position within a system of property relations, defining one’s location with a structure of domination, control, and exploitation. The group of people who share a similar position within the property relations of a society constitute a class. Their circumstances, resources, and opportunities are similar to those of others in the class, and they have common interests that are in opposition to members of some other classes. So class works as a social sorting process: individuals are tracked into one class or another through specific sociological mechanisms (schooling, parental attitudes, neighborhood). And it works to assign very different ranges of material outcomes to members of the various groups; working-class families wind up more poorly educated, less healthy, and more vulnerable to economic fluctuations than their counterparts in the landlord class, the financial elite class, or the capitalist class. Part of the challenge of developing a sociology of class involved identifying some of the concrete pathways of difference created by class with respect to specific opportunities – education, health, adequate nutrition, access to creative work, and other important social resources.
Status and consciousness are also part of the sociology of class. And, of course, there is the concrete sociological task of better understanding the lived experience of people who wind up in the various segments of the class system. Individuals develop specific features of mentality out of the experience they have in the class environments of their parents, their schools, and their workplaces. And these differences in turn give rise to differences in behavior -- consumer behavior, political behavior, and inter-group behavior. And members of a class may acquire a common perspective on their situation -- they may come to diagnose the social relations around them in a similar way, they may come to a common “class consciousness” that leads them to engage in collective action together.
Evidently, the groups that own capital and land have access to material resources that owners of labor power do not; so capitalists and landlords have social advantages lacked by proletarians. Proletarians gain access to material goods by selling their labor power to owners of capital and land; they become wage laborers. Class relations create substantial differences of material wellbeing and substantial inequalities of wealth and income. By controlling the wealth constituted by capital and land, these privileged classes are able to take a disproportionate share of society's wealth. The great modern social classes, in Marx's historical analysis, are the bourgeoisie (capital and land) and the proletariat (wage labor). In feudalism the great classes were the feudal aristocrats (owners of land and rights in the labor of serfs) and serfs (usufruct of small parcels of land, labor obligations to the lord).
Class and property are thus conceptually intertwined. An economic structure can be defined as a system for producing social wealth in which productive resources and the results of production are unevenly divided across different groups. Classes are the major social positions within an existing economic structure. Producers create wealth through their labor and creativity; property owners extract a part of this wealth through a system of social relations that privilege them. Another way of putting the point is to ask: where does the individual gain his/her income -- from the sale of labor time, from the sale or rent of physical assets, or from the sale or rent of expertise? Workers derive their income from the sale of their labor time; capitalists, financiers, and landlords derive their income from their ownership of physical and financial resources, and professionals, experts, and intellectuals derive their income from their possession of scarce expert knowledge and skills.
In nineteenth-century France we might have classified the population into land owners, capital owners, wage laborers, artisans, professionals (accountants, architects), intellectuals, government officials, civil service workers, small merchants, smallholding farmers, tenant farmers, landless workers, and lumpenproletariat. And these groups can be roughly triangulated according to their ownership of three major elements: labor power, valuable skills and knowledge, and economic assets (land, property, wealth). Within any society there are groups that fall outside the primary classes -- small traders, artisans, small farmers, intellectuals. But it is central to Marx's theory of class, that there is a primary cleavage between owners of the means of production and the direct producers, and that this cleavage embodies a fundamental conflict of interest between the two groups.
Classes, according to Marx, also constitute a system of exploitation: a system in which a substantial share of the fruits of social production are transferred from one group to another, through the normal workings of the social-property system. The producing class is exploited by the ascendant class: wealth is transferred from producers to owners. Serfs and lords, slaves and masters, workers and owners represent the primary classes of feudalism, ancient slavery, and nineteenth century capitalism. The proletariat produces surplus value, and the bourgeoisie gains ownership of this surplus through the workings of the property system, in the form of profits, interest, and rents. As Marx puts it in Capital:
He, who before was the money owner, now strides in front as capitalist; the possessor of labour power follows as his labourer. The one with an air of importance, smirking, intent on business; the other, timid and holding back, like one who is bringing his own hide to market and has nothing to expect but—a hiding.
Finally, the theory of class suggests the need for a theory of class consciousness: the ways in which members of distinct classes understand their roles in society, and the social relationships that largely determine their fates. Marx’s concept of ideology is intended to express the notion that large system of ideas serve a social function of concealing the conflictual nature of the property and class system in which people find themselves. The concept of false consciousness falls within this notion; members of a class possess false consciousness when they seriously misconstrue the nature of the social relations within which they live.
The explanatory thrust of the theory of class goes along the lines of a sociological hypothesis: people who have a similar location within a system of property relations will also develop other important similarities: similarities of thought, values, style, behavior, and politics, for example. And so Marx believed that structurally-defined classes of people were likely to further develop a similar class consciousness -- a similar framework of thought in terms of which they understand the social forces around them; and he expected that classes of people would come to share a signature framework of political motivation -- a set of ideas, interpretations, and values that would motivate them to engage in collective action together.