This document discusses the concept of federalism, which refers to a system of government that divides power between a central national government and several state or local governments. It outlines some key aspects of federalism such as the sharing and balancing of powers between federal and state governments. For example, the national government is responsible for national defense but not local issues like smoking laws. The document also examines different models of federalism and how the balance of power between federal and state governments has evolved over time in the United States.