Appropriation of the product of the work 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 work and took a share of the product of the exploiting classes by 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 “free” widespread exchange of goods, apparently equal in value, on the market. However, the use of a particular commodity, labor power, by its buyers, the holders of capital, would produce -somehow- an increase in value, a surplus value. Wealth, that is to say the product of work, would magically grow as a result of an endless and ascending cycle of exchanges capable of mobilizing the labor power for the production of what is exchanged.
However, under this appearance it is actually hidden that capital is nothing but a right to exploit labor power. And that the system as a whole is nothing but a more or less “automatic” way of generating new rights of exploitation for the holders of capital. The successful accumulation of capital is presented as a necessity for the members of the exploiting class who, in order to remain in it, must make capital grow. The system thus rewards with additional rights of exploitation those who have made the most efficient use of the capital they started with.
It is not commodity circulation which produces value. Nor does it mean that labor gives value to objects as it seemed to the classical economists. What happens is that the total value of commodities produced by an economy is nothing more than the socially necessary labor to produce them. And if capital is necessary to produce each one of them, it is because in every class society, a prior “right” is needed to be able to organize and dedicate a certain amount of labor power to the production of something and to exploit it legitimately.