Grand Admiral, First Order Central Command
Strategic/Regional/Oversector Command:
The highest level of command, typically run by a Grand Admiral or Grand Moff. Controls the movements of all troops, ships, and logistics in an entire region of space or entire theater of a war. At present due to the Empire's small size none have been established.
Sector Group:
A Sector Group is made up of all the forces that are required by a sector to maintain a reasonable level of defense and control. Typically this is several fleets and possibly many smaller independent units. Typically commanded by a Moff or Fleet Admiral (often the same person, except in sectors that see lots of action). At present there is only the Atrisian Sector Group, known as 'Divine Wind Command.'
Fleet:
A Fleet is a large collection of ships capable of operating as a single unit (though they are rarely deployed as such). Multiple fleets make usually make up a Sector Group, though an Oversector Command may also have fleets that report only to it. At present the Imperial Navy consists of three fleets. The Imperial Core Fleet, the Outer Rim Fleet, and the Reserve Fleet. A fleet is typically commanded by an Admiral or Vice Admiral, and generally consists of no less than four Star Destroyer-class vessels (but up to as many as 40).
Strike/Task Force:
The realities of warfare in space and maintaining a navy being what they are, it is rare to find a whole Squadron operational at the same time, much less a fleet. That truth, combined with the need for an intermediate force, larger than a squadron but smaller than a fleet, lead to the development of the Strike Force. Whereas fleets are primarily administrative organizations, a Task Force is a mission-focused unit, organized with a specific goal in mind. Some may be nearly as large as a fleet (but cobbled together from units from all over) while others may be barely larger than a Squadron.
The main difference between a Task Force and Strike Force is whether the unit is organized around a single (usually large) ship or around a general goal. The most common Strike Group is formed to protect a single Jun-class Carrier (and is known as a Carrier Strike Group). Occasionally, several Strike Force's may merged into a 'Combined Task Force.' In practice, however, the two terms are interchangeable and are the cause of much confusion. In all cases, typically commanded by a Vice Admiral or Rear Admiral.
Squadron:
The smallest administrative organization, a squadron consists of several ships of a class combined together. The name also tends to vary by class, with Star Destroyers and larger typically being formed in 'Divisions' instead. Squadron's are usually controlled by a Rear Admiral a Commodore.
2-6 Star Destroyers
4-8 Heavy Cruisers
4-12 Cruisers
6-20 Frigates/Corvettes
Star Destroyer+ - Commanded by a Captain or Senior Captain
Cruiser/Heavy Cruiser - Commanded by a Commander
Frigate - Commanded by a Lieutenant Commander
Corvette - Commanded by a Lieutenant
Bear in mind that when it comes to command, an Admiral picks a ship to be the 'flagship' of a squadron/fleet/task force/etc. That ship still has its own captain who directs the individual actions of that ship, whereas the flag officer is concerned with the larger picture.
The highest level of command, typically run by a Grand Admiral or Grand Moff. Controls the movements of all troops, ships, and logistics in an entire region of space or entire theater of a war. At present due to the Empire's small size none have been established.
Sector Group:
A Sector Group is made up of all the forces that are required by a sector to maintain a reasonable level of defense and control. Typically this is several fleets and possibly many smaller independent units. Typically commanded by a Moff or Fleet Admiral (often the same person, except in sectors that see lots of action). At present there is only the Atrisian Sector Group, known as 'Divine Wind Command.'
Fleet:
A Fleet is a large collection of ships capable of operating as a single unit (though they are rarely deployed as such). Multiple fleets make usually make up a Sector Group, though an Oversector Command may also have fleets that report only to it. At present the Imperial Navy consists of three fleets. The Imperial Core Fleet, the Outer Rim Fleet, and the Reserve Fleet. A fleet is typically commanded by an Admiral or Vice Admiral, and generally consists of no less than four Star Destroyer-class vessels (but up to as many as 40).
Strike/Task Force:
The realities of warfare in space and maintaining a navy being what they are, it is rare to find a whole Squadron operational at the same time, much less a fleet. That truth, combined with the need for an intermediate force, larger than a squadron but smaller than a fleet, lead to the development of the Strike Force. Whereas fleets are primarily administrative organizations, a Task Force is a mission-focused unit, organized with a specific goal in mind. Some may be nearly as large as a fleet (but cobbled together from units from all over) while others may be barely larger than a Squadron.
The main difference between a Task Force and Strike Force is whether the unit is organized around a single (usually large) ship or around a general goal. The most common Strike Group is formed to protect a single Jun-class Carrier (and is known as a Carrier Strike Group). Occasionally, several Strike Force's may merged into a 'Combined Task Force.' In practice, however, the two terms are interchangeable and are the cause of much confusion. In all cases, typically commanded by a Vice Admiral or Rear Admiral.
Squadron:
The smallest administrative organization, a squadron consists of several ships of a class combined together. The name also tends to vary by class, with Star Destroyers and larger typically being formed in 'Divisions' instead. Squadron's are usually controlled by a Rear Admiral a Commodore.
2-6 Star Destroyers
4-8 Heavy Cruisers
4-12 Cruisers
6-20 Frigates/Corvettes
Star Destroyer+ - Commanded by a Captain or Senior Captain
Cruiser/Heavy Cruiser - Commanded by a Commander
Frigate - Commanded by a Lieutenant Commander
Corvette - Commanded by a Lieutenant
Bear in mind that when it comes to command, an Admiral picks a ship to be the 'flagship' of a squadron/fleet/task force/etc. That ship still has its own captain who directs the individual actions of that ship, whereas the flag officer is concerned with the larger picture.