What is federalism?
It is a form of government where power is distributed at local/regional as well as national level
If we look at the American system, the Constitution and the creation of the USA very much reflected the struggle faced by the Americans in their quest for independence from Britain.
Thus power is spread out so that no one area of government becomes too powerful.
There is therefore the Federal Government in Washington DC. This comprises the Legislature- Congress, Executive- President and Judiciary- Supreme Court. The intention was to create a balance of power through the Separation of Powers. For example no member of the legislature can be a member of the Executive, unlike the British system.
As well as this each of the now 50 states has its own government structure in the form of Governors, state legislatures and state judicial systems. States' Rights and the independence of state powers is still a very important and sensitive part of the American political system.