Each of the social groups defined by the productive relations prevailing in a given society.
The division into classes proper to each mode of production corresponds to differentiated class interests and to different, more or less incomplete and distorted forms of consciousness of those interests, 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 rests.
The fundamental idea that permeates the whole "Manifesto" -namely:
that economic production and the social structure necessarily derived from it in each historical epoch constitute the basis on which the political and intellectual history of that epoch rests;
that, therefore, all history (since the dissolution of the primitive regime of common ownership 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 struggles.
Frederick Engels. Preface to the German edition of the Communist Manifesto, 1883