Grand Moff was the title given to the Regional Governors of Oversectors. These sectors, also called Priority Sectors, were locations that were of special interest to the Emperor. These included the Imperial capital of Coruscant and the Death Star. Some Moffs like Grand Moff Tarkin held authority over multiple Priority Sectors.
Grand Moffs received drastically increased authority, military forces, and funding in order to compensate for their enhanced position. The position of Oversector governor was the sixth highest in the Empire after Emperor, Executor, Grand Vizier, Imperial Ruling Council chief and Grand Admirals/Grand Generals. Technically, this made them comparable to Imperial Advisors as they both answered directly to the Emperor. While Imperial Advisors could appoint Moffs, only the Emperor himself could appoint Grand Moffs. The actual nature of the upper levels of the Imperial government was extraordinarily complicated, partially because the Emperor wanted those below him to be uncertain where their position truly stood.
Grand Moffs were personally appointed by the Emperor's will but the first of their number, Grand Moff Tarkin, was actually promoted by Ars Dangor after the submission of the Tarkin Doctrine. The Grand Moffs reported directly to the Emperor and competed for his favor.
The Grand Moffs were higher in authority than the Moffs but there is no indication that Moffs actually reported to the Grand Moffs. Instead, the Grand Moffs held direct control over their territories. Grand Moffs also had drastically different levels of power with Grand Moff Tarkin, having control over most of Oversector Outer, while Grand Moff Hissa likely only had control over the Kessel Sector. It is possible the Emperor weakened the title of Grand Moff after the disaster with the Death Star, though this may merely be circumstantial in Hissa's case.
In theory, the Imperial Advisors were more prestigious than the Grand Moffs and closer to the Imperial seat of power on Coruscant. Their direct access to the Emperor gave them less practical power than Grand Moffs, who often wielded supreme authority over their territories in a manner that brooked little opposition. This is demonstrated by the Grand Moffs' role in the Imperial Mutiny. Part of this may be because the Imperial Advisors had no military power by themselves while the Grand Moffs had their massive Oversector Fleets.The Grand Moffs of the Legacy era wielded quite different authority than the Mofference under Palpatine. The Grand Moffs of this era did not seem to act as Regional Governors but instead have seemingly replaced the Ruling Council of the Emperor. The heads of Imperial Intelligence, Supreme Commander, and head of the Imperial Mission were all Grand Moffs on the Council.
The title of Grand Moff, now inseparably identified with the Empire, was neither Palpatine’s nor Tarkin's invention, though both can be credited with resurrecting the title for Imperial use. In fact, the title dates back to the Expansionist Period (ca. 25000-20000 BBY), when the Republic frequently encountered many small satrapies and mini-empires. The warlords and satraps who joined the Republic instead of fighting it were permitted to keep their rule as proxies of the Republic, and their realms were granted the status of Allied Regions. Depending on the size of their empires, these satraps were given the title of either Moff or Grand Moff of an Allied Region. Over the ensuing millennia, most of the Allied Regions were divided into Republic sectors and gradually faded from the maps. The few Moffs and Grand Moffs that remained held largely ceremonial titles, and ultimately those titles, like so many of the remnants of the ancient Republic, were swept away during the Ruusan Reformation (ca. 1000 BBY). By the time of Palpatine, there were very few Allied Regions remaining, and most of those were small areas of space in the Core and Colonies, such the Ollonir Boundaries or the Botor Enclave.
The use of the title of Grand Moff was at one time so common that it was found in use in the Sith Empire in 3,623 BBY, and presumably in a role similar to that in the later Galactic Empire. One notable example of the Sith's use of the title was Grand Moff Kilran.
From the time of the Reformation on, the galaxy was organized into 1,024 regional sectors, which were themselves divided into their constituent subsectors. Under Republic law, the Supreme Chancellor could appoint a governor-general to coordinate military actions with the Senator of a troubled sector in a time of emergency. During the Clone Wars (22-19 BBY), Supreme Chancellor Palpatine frequently took advantage of this once-obscure law, appointing officers answerable directly to him as governors-general over both Republic worlds in crisis and Separatist worlds that had recently been liberated. When such actions met with criticism, Palpatine replied that with the passage of the Reflex Amendment (20 BBY), it was not just his right but his duty. In fact, the crisis gave him a good excuse to begin establishing a bureaucracy that was directly beholden to him.
The passage of the Sector Governance Decree (19 BBY) gave Palpatine the power to carry this process forward and appoint governors-general over the Republic’s 1,024 regional sectors, establishing a new permanent administrative class answerable to him only. With the inauguration of the Empire days later, the honorary title of Moff was announced for these new regional governors. The use of this title was calculated; many peoples of the galaxy still retained a sense of identity that had been derived from the traditions of the ancient Allied Regions, and they cheered the revival of this title in a new capacity as a check to a Senate most derided as corrupt. The first of these Moffs were installed within weeks, accompanied by a full regiment (2,304 soldiers) of stormtroopers.
Under the Sector Governance Decree, twenty of these new Moffs were also given responsibility for supporting the Republic's twenty Sector Armies, and their military territories, generally matching the sphere of influence of their respective Sector Armies, were dubbed oversectors. These oversectors straddled the boundaries of regional sectors and freed these twenty Moffs from the need to consult with the Senators representing these regional sectors. In the course of the Empire's further militarization (ca. 5 BBY), Moff Wilhuff Tarkin submitted a special holocommunique to Palpatine suggesting a reorganization of these oversectors as part of his Tarkin Doctrine. This was an attempt to cope with the problems of increasing Rebel activity across sector lines. Palpatine evidently approved of this suggestion, and acted accordingly; some of the previous oversectors - such as Imperial Center Oversector - were left in place, but others were expanded - Tarkin's own territory, the Greater Seswenna Sector, was enlarged into Oversector Outer. Others still disappeared entirely, replaced by a shifting number of oversectors comprised of systems in which Rebel activity was particularly pronounced. With the permanence of oversectors codified into law, the ancient title of Grand Moff was revived and awarded to those Moffs who controlled these oversectors. Tarkin was the first of these, and with this promotion to priority sector high commander of Oversector Outer came Palpatine's command to restart work on the Death Star project.
The rank suffered the first in a series of scandals when Grand Moff Trachta joined with others in an attempted coup d'état against the Emperor. The plot failed and the treacherous Grand Moffs were executed for their betrayal. This would not be the end of their plotting since the Emperor suspected (correctly) that Grand Moff Tarkin also held ambitions towards the throne.
During the Galactic Civil War, the Rebellion captured Grand Moff Ravik and intended to imprison him for trial when he was killed in a strange adventure involving the Charon. The mutated Grand Moff Ravik was eventually killed by Rebel Alliance agents.
The death of the Emperor resulted in a free for all amongst Imperial warlords for rule of the galaxy. One of the most powerful of breakaway warlords was Grand Moff Ardus Kaine. The successor of Grand Moff Tarkin created the powerful Pentastar Alignment that solidified Imperial power in the Outer Rim territories but removed a substantial portion of Coruscant's influence. Grand Moff Kaine would be killed during Operation Shadow Hand. Whether he was killed by Rebel forces or by the Emperor for treason is unknown.
Another act of Grand Moff treason occurred when Grand Moff Hissa united several powerful Moffs together into the Central Committee of Grand Moffs that attempted to usurp Ysanne Isard's rule over the galaxy. Despite having the forces of three Grand Admirals at their service, their coup failed with most sentenced with prison sentences. The exception to this was Rufaan Tigellinus, whom was executed by the Moffs for demanding a higher position amongst their ranks. The power of the Grand Moffs would be broken for the remainder of the Galactic Civil War.
It is unknown if any Grand Moffs survived until the period of the Emperor Reborn but it is noteworthy that at least some Moffs (which might include Grand Moffs) rebelled against the thought of a new Emperor being elected by the Advisory alone. These campaigned for their worlds to be given vote representation to allow them to control the process of selecting the next leader of the galaxy.
The position would be revived in the Legacy era with the Council of Moffs replaced by the Moff High Council. Presumably, this is the result of Imperial expansion. It is during this time that Grand Moff Morlish Veed and his consort Nyna Calixte collaborated with the Sith in yet another Moff born coup d'état against Emperor Roan Fel. The coup was supposed to place Morlish Veed on the throne but instead ends with his position unchanged in the Empire. Instead, Darth Krayt assumed the Imperial throne.
Wilhuff Tarkin's rank insignia
Alternate Grand Moff rank insignia
Grand Moff breast rank insignia
The Grand Moff's rank insignia was composed of six blue bars, three red, and three yellow. This indicated that the Grand Moff possessed a rank in the military higher than even that of an Admiral or General in the Imperial armed forces.
With the establishment of the second Galactic Empire, the insignia of Grand Moffs changed. The new insignia comprised two epaulets joined by a neck-brace-like adornment worn instead of the usual rank insignia.
The title of Grand Moff came from the title, "Grande Mouff", that was originally given to a (much changed) Tarkin from the rough, first, second and third draft drafts of Star Wars. In the former two drafts, Tarkin was a religious leader on Aquilae or Townowi; in the second and third drafts he was the Rebel commander of the Kesselian Dragoons on Yavin 4.
Due to events of Revenge of the Sith, Grand Moff Tarkin's promotion must have taken place during or perhaps even before the events of the film. This is due to the fact that Grand Moff Tarkin received a promotion directly as a result of the Tarkin Doctrine and was only assigned the Death Star project because of it. The Death Star is partially completed during the film and given Regional Governors are a product of the Clone Wars, it's entirely possible that this was an Old Republic decision rather than an Imperial one.
This is a complex problem given the Death Star Technical Companion was written with the idea of Grand Moff Tarkin being elevated during the Empire and the Death Star being constructed well into the Empire's reign.