Relationships between social classes to bring about the process of producing the means of subsistence of society. These relationships depend in turn on the connections that each class has with the means of production, particularly their appropriation or not, which reveals the central place that the form of property takes in every society.
For example, capitalist relations of production consist of the commercialization of labour-power, through the existence of those who own the means of production (capitalists) who need to buy labour power in order to produce and those who do not own (proletariat), obliged therefore to sell their capacity for labour. This relationship is not between equals, rather the capitalists only pay workers the equivalent of a part of their labour, appropriating the rest as profit, what defines this relationship as exploitation (due to the fact that the non-paid labour is the only source of profit for the capitalist class). Therefore, the formula of freedom of the market in reality alludes to freedom of exploitation (see Marx’s 1848 Speech on the Question of Free Trade).
While determined relations of production cannot exist without a certain level of development of the forces of production, the relations of production also influence the forces of production (see Engel’s brief Letter to Bloch from 1890).