Functionalist theory (also known as functionalism) and conflict theory are two of the major schools of thought in the study of sociology.
Functionalism argues that society is a self-stabilizing "organism" in which institutions and norms arise to fulfill needs and deficiencies in the overall functioning of human beings as a communal group. On the other hand, conflict theory posits that social behavior is driven by competition for scarce resources, which is the base posit of most popular economic theory. (It should be noted that conflict theory is not a singular school of thought; rather, it is a term used to categorize many approaches to sociology that center competition—but for the broad scope of this question, it can simply be understood in general terms.)
Generally speaking, as a capitalist nation, the US system is based on a blend of these theories in the style of Adam Smith's "invisible hand." It is assumed that while individuals and small entities (e.g., households and businesses) are competing for resources, society as a whole will even itself out to maintain a status quo.
This obviously does not pan out well in practice, as mega corporations form oligopolies and flourish, making a handful of individuals unthinkably rich and powerful while the working class is rapidly decreasing in mobility and economic power.