What does "Separation of Powers" mean?

The Articles of Confederation failed because they did not
give the central government enough power.  It had no power to tax, to raise troops for
an army, to control interstate commerce, and to stop states from printing money, among
many others.  So the framers of the Constitution knew that they had to create a strong
federal government, but at the same time make sure it did not become too powerful.  So
they incorporated the principles of Separation of Powers,
Checks and Balances, and
Federalism into the
Constitution.

The framers of the Constitution (those who
framed, or wrote, the Constitution) feared a powerful central, or national, government. 
In order to keep the national government from becoming too strong, they divided the
government into three branches, the legislative, the executive and the judicial.  The
framers then gave each branch separate, specific powers.  This is known as
separation of powers—that is, the
powers of the national government were divided among the three branches of
government.

The framers were also concerned about one
branch of government gaining too much power; therefore, they also established a system
of checks and balances.  Each branch
of government was given the ability to check the power of the other two branches of
government.

Finally, the framers of the Constitution were
concerned that a national government would dominate the state governments—that is, the
national government would have too much power and the state governments would have too
little power.  To keep this from happening, the framers established a government based
on federalism.  In this system, power
would be divided between the federal government (that is, the national government) and
the state governments.  Some powers would be given or delegated only to the federal
government, some powers would be reserved to the states, and some powers would be
shared—both the federal government and state governments would have that
power.