A federal system of government is characterized by the constitutionally-mandated division of political authority between the national government and sub-national territories, such as states or provinces. While being under one central government, each sub-unit maintains a certain level of political autonomy to better serve its population.
The division of powers between the national and sub-national governments can only be changed or taken away by amending the constitution. This is in contrast to the delegation of powers in a unitarian government. In the latter system, the national government may delegate or take away political power from its sub-units, usually through legislation.
A federal system's constitution protects the very existence of its political sub-units. Neither the national government nor its sub-units can destroy each other. In a unitarian system, political sub-units may be abolished or reorganized by the national government through legislation, which is not possible in a federal system of government.
In the case of the United States, the division of powers between the national government and its state governments is protected by the 10th Amendment to the U.S. Constitution.
Other countries with federal systems of government include Canada, Brazil, Australia, India, Argentina, Austria, Germany, Malaysia, Mexico, Nigeria, Switzerland and Venezuela.