There are six ranks of General in the United States Army.
Brigadier General, one silver star
Major General, two silver stars
Lieutenant General, three silver stars
General, four silver stars
General of the Army, five silver stars.
There have only been five men in history with the rank of General of the Army: Eisenhower, Marshall, MacArthur, Arnold, and Bradley. The rank insignia features a circle of five silver stars.
A General of the Armies, however, outranks these five men. This rank has no official insignia but is commonly referred to as a six star general. The Army at one time proposed to have a circle of five silver stars with a single silver star within the circle.
Washington and Pershing are the only two men who have held the rank of General of the Armies. Pershing was elevated to that rank in 1919 before there was ever a five star general. Pershing, on his own, adopted four gold (vs silver) stars to represent his rank.
Congress later purposely made a General of the Army subordinate to a General of the Armies, which brings the unofficial six stars into play.
Congress retroactively awarded General of the Armies rank to George Washington intending that he should never be outranked. Since Washington's date of rank is over one hundred years earlier, he outranks Pershing.