Supreme Commander, Grand Admirals and Warlords
Grand Admirals are an innovation of Palpatine. They are not necessarily in command of any particular Sector or Fleet but, similar to Grand Moffs, are put in charge of specific areas or specific missions as needed. They exist outside the traditional Imperial/Old Republic Naval hierarchy and answer only to The Emperor; their authority is his authority and disobeying their commands carried the crime of High Treason - which in the Imperial era was codified in law as an attempt to overthrow The Emperor. Summary execution (without trial) was the penalty.
Unlike Grand Moffs, Grand Admiral rank was permanent and would not cease to exist once their mission was deemed complete. A Grand Admiral outranked a Grand Moff in the Imperial Hierarchy.
Supreme Commander of the Republic forces was typically promoted from the High Admiral or Field Marshal ranks of Republic service. He had overall command of all Republican forces. Under The Empire the rank became mostly bureaucratic in nature as Palpatine would not risk a rival who had such forces under his command.
Warlord was a ceremonial title given to victorious commanders under The Empire. Only the greatest victories that brought great wealth and/or eliminated a major threat to The Empire could result in the title being granted. The recipient was allowed to wear gold piping on their uniforms. It also carried a substantial yearly allowance from the Imperial coffers.