Final answer:
The theory referring to the division of society along class lines, with the upper class holding significant power, is known as conflict theory.
Step-by-step explanation:
The theory that states society is divided along class lines and the only group with real power is the upper class is known as conflict theory. This sociological perspective highlights that the upper class not only has power and control over their own lives but also has significant influence over the lives of others in society. Originating from the works of Karl Marx, this theory has been further elaborated by C. Wright Mills and G. William Domhoff, who discuss the power elite and the ability of this small group of individuals to dominate social and political realms through wealth and social networks.