United States Constitution
We the people of the United States, in order to form a more perfect union, establish justice, insure domestic tranquility, provide for the common defense, 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 Constitution of the United States of America
The United States Constitution is the defining document of the government of the United States. Unlike the Declaration of Independence, which only informs legal decisions, the Constitution is the "highest law of the land."
The Constitution enumerates specific rights and powers the government (and its citizens) possess. It is the standard by which new legislation is passed. If a law is deemed "unconstitutional" by the Supreme Court, for example, it can be overturned.
The Constitution has been amended several times over its history, though it is purposely difficult to do so. The most notable amendments are the first ten, collectively known as the Bill of Rights, which guarantee freedom of speech, due process, and other basic civil rights. Without the Bill of Rights the Constitution would not have been ratified by the original states.