United States Government/The Federal System
The United States is exactly that--a Union of states. Each state has its own individual powers. However, that does not mean that the states have power to legislate on all matters. The Constitution of the United States spells out the powers of the federal government and of the "several states." The Union government (known as the federal government) has its own fields of legislation, and if federal legislation conflicts with the state laws, the federal legislation prevails. If this occurs, the state must defer to the federal government. The alternative, that any state may at any time leave the Union and thus be free from Union interference in the state's internal affairs, was tried during the American Civil War.
Types of Federalism 
There are two types of federal systems. The first, dual federalism, holds that the Union and the state are equal; under this view of federalism, the Union government only has the powers expressly granted to it, while the states retain all other powers. The second view, cooperative federalism, states that the federal government is definitively superior to the state government, and the federal government should stretch its powers as far as possible. The US is a Union that does not completely fit either definition. The type of federalism in effect really depends on who is in power at the time. In any case, the Constitution prevents stretching federalism too far in either extreme.
All powers retained by the states are known as reserved powers. Those specifically granted only to the Union government are known as enumerated powers. Finally, matters over which both the Union and the state governments have control are known as concurrent powers.
The Tenth Amendment provides that the Union government has only the powers expressly designated to it by the Constitution, and the states control all other matters.
Enumerated powers relate to the following:
- Regulation of interstate and foreign commerce
- Immigration and Citizenship
- Weights and Measures
- Copyrights and Patents (Special rights granted to authors and inventors relating to the writing or discovery)
- Crimes at sea
- Declaration of War
- Control of the District of Columbia
- Federal Courts
- "Necessary and Proper"- The necessary and proper clause allows the Union government to make laws necessary to utilize other powers. For instance, this clause allows the government to jail persons who violate laws created under the aforestated powers.
Reserved Powers 
As has been mentioned, the states have control over all matters not controlled by the Union government. Some of these matters include:
- Police protection
- Control local governments
- Regulate individual and corporate activities in order to protect and enhance public health, welfare, safety, morals, and convenience.
Concurrent Powers 
Several powers belong concurrently to the Union and the state governments. These include:
- Make and enforce laws
- Establish courts
- Spend money for general welfare
- Take private property for public purposes, with just compensation.
- Charter banks and corporations