Appropriation of the product of the labor of one social class by another.
Historically, exploitation has mainly taken extra-economic forms. The ancient slave-owner, the feudal lord or the Chinese mandarin organized social labor and took a share of the product of the classes they exploited by means of exaction and the threat of the use of violence.
Capitalism is the first mode of production in which exploitation takes place by exclusively economic means, in the form of a generalized "free" exchange of commodities, apparently equal in value, on the market. However, the use of one particular commodity, labor power, by its buyers, the owners of capital, would produce -no one knows how- an increase in value, a surplus value. Wealth, that is, the product of labor, would grow magically as a result of an endless, ascending cycle of exchanges capable of mobilizing labor power for the production of what is exchanged.
However, what this appearance actually hides is that capital is nothing but a right to exploit labor power. And that the system as a whole is nothing more than a more or less "automatic" way of generating new rights of exploitation for the owners of capital. The successful accumulation of capital presents itself as a necessity for the members of the exploiting class, who, in order to remain in it, must make their capital grow. The system thus rewards with more rights of exploitation whoever made the most efficient use of the capital they started out with.
It is not circulation that produces value. Nor is it that labor adds value to objects, as it seemed to the classical economists. It is that the value of the commodities produced by an economy is nothing but the labor socially necessary to produce them. And if capital is needed to produce each of them, it is because in every class society a prior "right" is required in order to organize a certain amount of labor power, devote it to the production of something and exploit it legitimately.