Are there other people or groups that make important decisions for the United States? Who are they & what do they do? (Please help )

The Constitution of the United States divides the federal government into three branches to make sure no individual or group will have too much power:

Legislative—Makes laws (Congress, comprised of the House of Representatives and Senate)
Executive—Carries out laws (president, vice president, Cabinet, most federal agencies)
Judicial—Evaluates laws (Supreme Court and other courts)
Each branch of government can change acts of the other branches:

The president can veto legislation created by Congress and nominates heads of federal agencies.
Congress confirms or rejects the president's nominees and can remove the president from office in exceptional circumstances.
The Justices of the Supreme Court, who can overturn unconstitutional laws, are nominated by the president and confirmed by the Senate.