How is the power to govern shared under the principle of federalism?

Federalism is the system in which (in the case of the US)
the Constitution divides powers between the state governments and the federal
governments.  By doing this, the Constitution gives each of these levels of government
their own power and prevents either level from completely dominating the
other.

Under the Articles of Confederation, the states
essentially had all the power to govern.  The national government had no power that the
states did not give it.  In such a system, the power to govern is not
shared.

Under the Constitution, both the federal government
and the state governments have defined powers.  In that way, the power to govern is
shared.