Marx argued for a mass uprising of a self-conscious industrial proletariat. When they were completely alienated from the means of production, they would overthrow the bourgeoisie, their opressors. But this would happen at an advanced stage of industrialization.
Lenin, on the other hand, adapted revolutionary Marxism to realities in Russia, which was not at the same developmental stage of industrialization, and had an enormous peasant class, which tended to be conservative. Marx had referred to French peasants as "sacks of potatoes," believing that they would never be a self-conscious class. Lenin thus concluded that a communist revolution would be brought about not by a massive worker's revolt begun by workers themselves, but by a revolutionary vanguard of intelligentsia led by a man like himself.