Ruling class
In Marxian political economics, the ruling class refers to that segment or class of society that has the most economic and political power. Under capitalism, the ruling class -- the capitalists or bourgeoisie -- consists of those who own and control the means of production and thus are able to dominate and exploit the working class, getting them to labor enough to produce surplus-value, the basis for profits, interest, and rent (property income). This property income can be used to accumulate more power, to extend class domination further. The economic power of the ruling class gives it extraordinary political power, so that state or government policies almost always reflect the perceived interests of that class.
The sociologist C. Wright Mills argued that the ruling class differs from the power elite. The latter simply refers to the small group of people with the most political power. Many of them are politicians, hired political managers, and military leaders.
In other modes of production, there are other ruling classes: under feudalism, it was the feudal lords, while under slavery, it was the slave-owners.
Most stable groups of social animals (including humans) have a visible and invisible "ruling class". The decision makers in the group may change according to the decision-type and/ or the time of observation. For example, it used to be assumed that modern societies were patriarchal and the elders dominated the real decisions. Many market-led economies focus on the decision makers of each particular (assuredly minor) market sector, who may in fact be children or women.