From Ellen Frank’s reply on Dollars & Sense a few years back:

In the early 19th century, as capitalist industrialization transformed Western European society, both advocates and critics of the emerging social order discussed it in similar terms. To early commentators, society was divided into broad social classes—landlords, laborers, capitalists—defined by their power over economic resources. Analyzing this new capitalist system entailed understanding the creation and distribution of the social surplus—production beyond what was necessary to feed and house the workers—since this surplus provided rents and profits for the non-laboring classes.
By mid-century, thinkers like Karl Marx had turned this language into a powerful critique of capitalism. Marx argued that the politically disenfranchised working classes were victimized in industrial factories, the fruits of their labor stolen and transformed into profits with which the capitalist class created ever-more powerful tools of exploitation. Socialist, communist, and other radical movements demanded political and economic change, some calling for revolution and an end to private ownership of the means of production.

Read rest here.