@boomerang,
http://en.wikipedia.org/wiki/General_of_the_Army_%28United_States%29
General of the Army (GA)[1] is a five-star general officer and is the second highest possible rank in the United States Army. A special rank of General of the Armies, which ranks above General of the Army, does exist but has only been conferred twice in the history of the Army. A General of the Army ranks immediately above a general and is equivalent to a Fleet Admiral and a General of the Air Force; there is no established equivalent five-star rank in the Marine Corps, nor the other three Federal uniformed services: the Coast Guard, PHSCC, and NOAA Corps). Often referred to as a "five-star general," the rank of General of the Army is reserved for war-time use only and is not currently active in the U.S. military.
http://www.historyguy.com/5-star-military.htm
In December of 1944, in the midst of World War Two, the new Five-Star officer rank was created, allowing generals and admirals to place a total of five stars on their uniforms and flags. In all, four Army generals, four Navy admirals and one Air Force general have held this rank.
The Army's Five-Star Generals:
General George C. Marshall
General Douglas MacArthur
General Dwight D. Eisenhower
General Omar N. Bradley
The Navy's Five-Star Fleet Admirals:
Admiral William D. Leahy
Admiral Ernest J. King
Admiral Chester Nimitz
Admiral William F. "Bull" Halsey
The Air Force Five-Star General:
General Henry "Hap" Arnold --Note: General Arnold actually was awarded this rank twice. In 1944, he received his fifth star while the air force was still part of the Army. It was then known as the U.S. Army Air Forces (AAF). After the AAF separated from the Army in 1947 and was renamed the United States Air Force, Arnold became the new service's only five-star General of the Air Force.