Each of the social groups defined by the productive relations prevailing in a given society.
To the division in classes proper of each mode of production correspond differentiated class interests and different forms, more or less incomplete and deformed forms of self-consciousness, which are expressed through the different ideologies present in each epoch.
The concept of class -and with it that of class struggle- is the premise on which the possibility of a materialist critique of history stands.
The fundamental idea which permeates the whole "Manifesto" -namely that the economic production and the social structure necessarily derived from it in each historical era constitute the basis on which the political and intellectual history of that era rests; that, therefore, all history (since the dissolution of the primitive regime of common property of land) has been a history of class struggle, of struggle between exploiting and exploited, dominant and dominated classes, in the different phases of social development; and that this struggle has now reached a stage where the exploited and oppressed class (the proletariat) can no longer emancipate itself from the class that exploits and oppresses it (the bourgeoisie), without emancipating, at the same time and forever, society as a whole from exploitation, oppression, and class struggle.
Frederick Engels. Preface to the German edition of the Communist Manifesto, 1883