We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.
-- Preamble to the U.S. Constitution
The U.S. Constitution, by its very terms, is "the supreme Law of the Land." Art. VI. Drafted at the Constitutional Convention in Philadelphia in 1787 and subsequently ratified by the thirteen original states (officially taking effect on June 21, 1788, when Rhode Island became the ninth state to consent to the new government), the Constitution is the source of all federal government powers and protects the fundamental rights and liberties of Americans in all fifty states. The Bill of Rights, which refers to the first ten amendments to the Constitution, was proposed by the first Congress and subsequently ratified by the states, taking effect on December 15, 1791. Additional amendments (which currently total 27) have been added over the more than two centuries that the Constitution has been in existence. Indeed, the U.S. Constitution generally is considered the world's oldest written national constitution.
Interestingly, the U.S. Constitution also is the world's shortest written national constitution, totaling approximately 4,400 words. The Constitution is divided into seven Articles. Article I creates and describes the legislative branch of the national government, declaring "All legislative Powers herein granted shall be vested in a Congress of the United States, which shall consist of Senate and House of Representatives." Art. I, Sec. 1. Article II creates and describes the executive branch of the national government, declaring "The executive Power shall be vested in a President of the United States of America." Art. II, Sec. 1. The judicial branch of the national government is created and described in Article III, which specifically establishes "one supreme Court" and authorizes Congress to establish "inferior Courts." Art. III, Sec. 1. The amendment process is set forth in Article V. The ratification process is set forth in Article VII. Articles IV and VI contain a variety of different but important provisions.
For a general section-by-section description of the U.S. Constitution, see here (from the U.S. Senate website).
For a scholarly annotation of the U.S. Constitution, see here (from the U.S. Congress website).
The theory of separation of powers and checks and balances that underlies the structure of the U.S. Constitution was set forth most famously by James Madison in The Federalist Papers No. 51. Here is a key excerpt:
But the great security against a gradual concentration of the several powers in the same department, consists in giving to those who administer each department the necessary constitutional means and personal motives to resist encroachments of the others. The provision for defense must in this, as in all other cases, be made commensurate to the danger of attack. Ambition must be made to counteract ambition. The interest of the man must be connected with the constitutional rights of the place. It may be a reflection on human nature, that such devices should be necessary to control the abuses of government. But what is government itself, but the greatest of all reflections on human nature? If men were angels, no government would be necessary. If angels were to govern men, neither external nor internal controls on government would be necessary. In framing a government which is to be administered by men over men, the great difficulty lies in this: you must first enable the government to control the governed; and in the next place oblige it to control itself.
The Federalist Papers is a collection of essays written by James Madison, Alexander Hamilton, and John Jay during the New York state ratification debates to encourage adoption of the new constitution. It is arguably the most important work of American political philosophy.