Put simply, the bourgeoisie sought to lower, or keep constant, wages and thereby increase profits while the proletariat would seek to improve their living conditions by seeking higher wages. The two interests of these two classes were opposed and could not be solved within the confines of capitalism. This was one of the many contradictions that eventually lead to the replacement of capitalism by socialism. For Marx these irreconcilable interests were expressed in class struggle. Upon hearing these words many will simply assume that what is meant is simply violent struggle between opposing classes.
Violent struggle is, however, only one manifestation of class struggle.
Class struggle can be anything from the activities of a political party to the writing of a letter to the local paper. Perhaps the best way to define class struggle is to define it as social action that results from the different interests of classes and which is necessary according to Marxists in order for class consciousness to develop. Class struggle is thus essential with regard to any transition to socialism. Marx was also concerned with explaining the origins of class and of classes. Other historians and commentators were prepared to take for granted the existence of particular classes, almost as if these classes had always existed.