Class warfare is a long-used term to describe social and political conflicts between classeses - groups of disproportionately different financial status. Financial status includes all assests a person has ownership of.
Money is the way modern societies quantify the work of its society. Since many economic systems have historically allowed money to be the property of individuals, these people can retain to their own dictates a power over those of lesser financial means - particularly those of the working class, who live in service of the upper class if we define the upper class meaning those with the means of production and the working class as those who work for the upper class.
Thus a socio-political imbalance exists between individuals of extreme wealth, and those with little or no wealth. The interests of those in whom power is established will often dramatically conflict with the interests and needs of the working class.
Furthermore, since the mechanisms by which wealthy peoples will increase or maintain their financial status are related and similar to another, they tend to develop similar political needs, and act in concert as a political group to those ends.
This is of course, is in direct contradiction to the wants, and often needs, of the greater majority. The only means by which people have in modern times made progress against this imbalance has been through democracy, and the institutionalized respect and protection of personal liberties.
Fundamentally, there is little difference between the class warfare that existed between the Victorian era monarchy and the common public, and a modern Corporation and its workers, who may own part or most of the corperation.
-- Unionss are a modern incarnation of public will in class warfare. Representing a political group of like minded working people, unions have revolutionised health and safety standards in industrial economies. Corporations are companies that exist as perpetual entities. Their function is as a vehicle for business enterprise, while transcending the bounds of mortality and liability that accompany an individual-owned enterprise.
See also: class envy, exploitation, exploitation of the working class, labor struggles, classes, sharecropping system, taxation class struggle class conflict