The Communist Manifesto

The history of all societies so far is the history of class struggles.
Freeman and slave, patrician and plebeian, baron and serf, master and journeyman, in a word, oppressor and oppressed, were in unceasing opposition to each other, they waged an incessant struggle, sometimes veiled, sometimes open, a struggle that ended each time with a rebel transformation of the whole society or by the joint destruction of the struggling classes.
The modern bourgeois society that emerged from the destruction of feudal society did not abolish class antagonisms, it only replaced the old ones with new classes, new conditions of oppression, new forms of struggle.
However, our age, the age of the bourgeoisie, is characterized by the fact that it has simplified class antagonisms. The whole of society is more and more divided into two great rival camps, into two great classes, which are directly opposed to each other: the bourgeoisie and the proletariat.
From the serfs of the Middle Ages came the free citizens of the first cities. The first elements of the bourgeoisie developed from this urban population.