Exploitation is the forced appropriation of the surplus value of workers. It takes different forms under different class systems and continues to exist in lower stages of Socialism. It disappears only in communist society.
In Capitalist Society
Under capitalism, what value the proletariat brings to the business, the bourgeoisie can only pay a fraction of that value. The wage that is paid represents the cost of living for the wage worker and the rest of the value goes to the bourgeoisie as profit. Proletarians are also exploited when buying commodities which will likely be priced higher than the cost of its production.
In Socialist Society
Socialism as the transitional stage between capitalism and communism will inherit many elements of capitalism. Exploitation will still persist but will generally be lower in quantity than exploitation under capitalist society. In a state enterprise, workers may be paid a fraction of the value brought to the enterprise but compared to a capitalist enterprise, more of the extracted surplus value may circulate as funding for public services and so the extracted surplus value eventually comes back to the worker. Cooperatives also have potential to exploit non-members through commodity exchange or through externalities such as the creation of pollution.