What best describes the constitution of the united states of america?

It is best described as a statement of rules and procedures for governing

Purpose of US Constitution
The Constitution is the document that created the present government of the United States. It was written in 1787 and went into effect in 1789. It establishes the three branches of the U.S. government, which are The executive branch (headed by the president), The legislative branch (the Congress), and The Judicial branch: The supreme court (and other federal courts). The bill of rights:explains the basic rights of all American citizens.

The primary purpose of the Constitution is to provide the framework which establishes the proper functions of government, and as a design document indicating how each section of government is to work (as well as the division of responsibilities amongst the parts of government). It is a "blueprint" document, which is less concerned with ideals (though, it does touch on some), and more concerned with functionality.