What did Karl Marx say about surplus value?

Karl Marx used the term of surplus value in his analysis of the economics of his time. For Marx, capitalists exploited workers by paying them just enough to keep them alive and working. He saw workers' wages as just above the subsistence level. Marx believed that the difference between wages paid to workers and the price of the goods they produced was "surplus value". Marx did not see factory owners contributing anything towards production. The surplus value created by the process described above the capitalists took as profit for themselves. The key to this was that Marx saw a time when the workers could not afford to purchase the goods they produced. Production led to over production and brought on depressions.