Surplus value
If you get paid 5$ an hour to make toasters and you make one toaster per hour and the owner sells that toaster for 20$ the surplus value the owner makes off you is 15$
Toaster 20$
labor 5$
_________
Surplus value 15$
Surplus value is a concept created by Karl Marx in his critique of political economy, where its ultimate source is unpaid surplus labor performed by the worker for the capitalist, serving as a basis for capital accumulation.
The German equivalent word "Mehrwert" means simply value-added (an output measure of the net increase in product wealth), but in Marx's value theory, the extra or surplus-value has a specific meaning, which is not the new value added to the output of products, but rather the amount of the increase in the value of capital upon investment, i.e. the yield or increment in value, regardless of whether it takes the form of profit, interest or rent.
Marx himself regarded the reduction of profit, interest, and rent income to surplus-value, and surplus value to surplus labour as one of his greatest theoretical achievements.
For Marx, the gigantic increase in wealth and population from the 19th century onwards was mainly due to the competitive striving to obtain maximum surplus-value from the employment of labor, resulting in an equally gigantic increase of productivity and capital resources. To the extent that increasingly the economic surplus is convertible into money and expressed in money, the amassment of wealth is possible on a larger and larger scale (see capital accumulation and surplus product).
http://en.wikipedia.org/wiki/Surplus_value