What is federalism in the US political system?

Federalism is a system of government in which a written constitution divides power between a central government and regional or sub-divisional governments. Both types of government act directly upon the people through their officials and laws. In the USA this is seen through the state system. In the UK we have a quasi-federal state. 

There are many types of federalism. Co-operative federalism assumes that the two levels of government have equal powers. The opposite is dual federalism this assumes that the two levels are functioning separately and have different levels of power.