What do karl marx and friedrich engels mean by the term class conflict?

Karl Marx and Friedrich Engles class conflict theory states that there are two classes and they have a perpetual conflict due to limited resources in society.  They claim that the social order is maintained by power and domination; not conformity and consensus.  The two classes that Marx describes are "the bourgeoisie" and the "proletariat".  The bourgeoisie controls the capital and the proletariat are the laborers.