"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States, respectively, or to the people."
"The ebb and flow of Tenth Amendment Jurisprudence reflects the delicate constitutional balance created by the Founding Fathers. The states ratified the Constitution because the Articles of Confederation created a national government that was too weak to defend itself and could not raise or collect revenue. Although the federal Constitution created a much stronger centralized government, the Founders did not want the states to lose all of their power to the federal government, as the colonies had lost their powers to Parliament. The Tenth Amendment continues to be defined as courts and legislatures address the balance of federal and state power."
http://legal-dictionary.thefreedictionary.com/Tenth+Amendment 
