A General usually has overall command of a whole army. His title comes from the Latin word generalis that meant something pertaining to a whole unit of anything rather than just to a part. As a military term General started as an adjective, as in Captain General indicating the Captain who had overall or "general" command of the army.
Before the Sixteenth Century armies were usually formed only when needed for a war or campaign. The king would be the commander but he might appoint a Captain General to command in his name. Later, when the title of Colonel became popular some kings called their commanders Colonel General. The British Army stopped using the Captain part of the title by the Eighteenth Century leaving just General as-the top commander. Some nations still use the Colonel General rank, among them the Soviet Union and East Germany. The king or his Captain General would often be away from the army since they had interests elsewhere so the job of actually running the army fell to the Captain General's assistant--his lieutenant--the Lieutenant General. This was not a permanent rank until the Seventeenth Century. One of the Colonels might be appointed Lieutenant General for a particular campaign or war but he would still command his own regiment. Since he might also be Captain of a company in his regiment, it was possible for one man to serve as Captain, Colonel and General simultaneously.
The army's chief administrative officer was the Sergeant Major General who was also appointed for the particular campaign or war. He would be an experienced soldier, possibly a commoner, who served as chief of staff. For much of his administrative work he dealt with the regimental Sergeant Majors, thus his title meant "overall" or "chief" Sergeant Major. His duties included such things as supply, organization, and forming the army for battle or march. Here again, as with the regimental Sergeant Major, a loud, commanding voice was a key requirement. As the General ranks became fixed during the Seventeenth Century the Sergeant portion fell away leaving the title as Major General. We can see this trend in England where in 1655 Oliver Cromwell, who ruled that nation temporarily as Lord Protector, organized the country into eleven military districts each commanded by a Major General.
The Lieutenant General and Sergeant Major General dealt directly with the Colonels who lead the regiments making up the army. When there got to be too many regiments for the two generals to handle effectively they organized battle groups or brigades, usually composed of three or more regiments. Brigade comes from the Florentine word brigare that in turn came from the Latin briga, both of which referred to fighting or strife. The brigade's commander was the Brigadier, who in some armies later became Brigadier General.
When our Army started in 1775 the Continental Congress commissioned George Washington General and Commander-in-Chief. He and his Major and Brigadier Generals wore various colored ribbons to show their ranks. There were no Lieutenant Generals in that army. In June 1780 General Washington ordered the Major Generals to wear a uniform that included two gold epaulettes with two silver stars on each epaulette. Brigadier Generals were to wear gold epaulettes with one silver star on each. General Washington might have chosen the stars because the generals and admirals of the French forces serving in that war wore stars. Another story has it that he was inspired by the stars in our new flag. The General's stars, then, are the oldest rank insignia still in use by our armed forces.
General Washington was the first to wear three stars when he became the nation's first Lieutenant General in 1798. After he died in 1799 there was not another Lieutenant General until 1855. The three stars appeared again, however, by 1832 as the insignia of the Major General who commanded the Army. In 1855 Congress honored Winfield Scott for his service as commanding general since 1841 and for his accomplishments in 1847 during the war with Mexico by making him a Brevet Lieutenant General. He held that rank until he retired in 1861. The next Lieutenant General was Ulysses S. Grant in 1864. Two years later he became the first General of the Army of the United States and chose four stars as his rank insignia. When Grant became President in 1869 he appointed William T. Sherman General of the Army and Phillip H. Sheridan Lieutenant General. Sherman changed the rank insignia in 1872 to a gold embroidered coat of arms of the United States between two silver stars. After Sherman retired in
1884 there was not supposed to be another General of the Army but in 1888 Congress relented and permitted the President to promote Sheridan who died two months later. Congress allowed another Lieutenant General promotion in 1895, one in 1900, five between 1903 and 1906, two in 1918 during World War I, one in 1929 and then no more until 1939. Our Army has been supplied with Lieutenant Generals since, as has the Marine Corps since 1942 and the Air Force since 1947.
There were no more full Generals after Sheridan died in 1884 until 1917 when Tasker H. Bliss, the Army Chief of Staff, and John J. Pershing, the commander of the U.S. forces in France during World War I, went from Major General to General (emergency) so they could have ranks equal to the allied commanders with whom they dealt. They changed the rank insignia back to four stars. In 1918, Peyton C. March also became a General.
In 1919 Congress honored Pershing for his wartime service by permitting the President to promote him to General of the Armies of the United States, which he held until he retired in 1924. He chose his own insignia, which was four stars. Nobody else has received that rank during his lifetime. In 1976 Congress authorized the President to posthumously appoint George Washington General of the Armies of the United States and specified that he would rank first among all officers, of the Army, past or present.
Congress did not allow the promotion of any more full Generals from 1918 to 1929 when the Major General chosen to be Chief of Staff also became a General so he could have a rank equal to the Chief of Naval Operations. Promotions for other Generals did not come until World War II, with the exception of a permanent promotion to General for Generals Bliss and March in June 1930. The Army still has several Generals, the Marines have had at least one General since 1945 and the Air Force, which started with three in 1947, also has several.
During World War II our Army got so big that even full Generals were not enough so in 1944 Congress created the new rank of General of the Army and specified five stars as its insignia. Congress did not revive the General of the Army rank held by Grant, Sherman and Sheridan. The World War II Generals of the Army were in a separate category from the Civil War Generals of the Army. In December 1944 the President appointed George C. Marshall, Douglas MacArthur, Dwight D. Eisenhower and Henry H. Arnold Generals of the Army. In 1949 Arnold's title became General of the Air Force. Omar N. Bradley got his fifth star in 1950.
As to the question of Pershing being a six-star general, there can be no answer unless Congress creates the General of the Armies rank again and specifies the insignia. Pershing does rank ahead of the Five-star Generals, he comes right after Washington, but he chose his own insignia and he never wore more than four stars.