How were worker exploited according to Marx's theories?

Basically Marx's theory was that under capitalism the means of production were owned by just a few, wealthy capitalists. The workers had to try to sell their labour power to the capitalists so that they may survive. When a worker goes to work for a capitalist they enter into a metabolic exchange with nature and produce exchange value. The capitalist takes this value and gives only a portion of it back to the worker, the rest of the value, or wealth, is taken by the capitalist as surplus value or profit.