How did Marx and Engels define the proletariat?

How did Marx and Engels define the proletariat? The bourgeoisie? Why did Marxists come to believe that this distinction was paramount for the understanding of history? What stepds did Marx and Engels believe would lead to a classless society?

How did Realism differ from Romanticism, and how did Realism reflect the economic and social realities of Europe during the middle decades of the nineteenth century?