Staff (military)
Manpower or personnel: The manpower or personnel officer supervises personnel and administration systems. This department functions as the essential administrative liaison between the subordinate units and the headquarters, handling personnel actions coming from the bottom up (such as a request for an award to be given to a particular soldier) or from the top down (such as orders being received from the army level directing that a particular soldier be reassigned to a new unit outside the command). In army units, this person is often called the Adjutant. The S-1 also works with the postal mailing office, and deals with awards and ranks as well.

Intelligence, security, and information operations: The intelligence section is responsible for collecting and analyzing intelligence information about the enemy to determine what the enemy is doing or might do, to prevent the accomplishment of the enemy's mission. This office may also control maps and geographical information systems and data. At the unit level, the S-2 is the unit's security officer, and the S-2 section manages all security clearance issues for the unit's personnel. Other duties of the S-2 often include intelligence oversight and physical security.

Operations: The operations office may include plans and training. The operations office plans and coordinates operations, and all things necessary to enable the formation to operate and accomplish its mission. In most units, the operations office is the largest of the staff sections and considered the most important. All aspects of sustaining the unit's operations, planning future operations, and additionally planning and executing all unit training, fall under the responsibility of operations. The operations office is also tasked with keeping track of the weekly training schedules. In most military units (i.e., battalion, regiment, and brigade), the operations officer carries the same rank as the executive officer (XO), but ranks third in the unit's chain of command while the other staff officers are one rank lower. For example, in a battalion, the S-3 would hold the rank of major (like the battalion XO), while the remaining staff officers are captains or lieutenants.

Logistics: The logistics office is responsible for managing the wide scope of materiel, transport, facilities, services and medical/health support:

Design, development, acquisition, storage, distribution, maintenance, evacuation, and disposition of materiel.
Transport of personnel and materiel.
Acquisition or construction, maintenance, operation, and disposition of facilities.
Acquisition or furnishing of services.
Medical and health service support.
By NATO doctrine, the logistic staff is tasked with overseeing logistic aspects and principles, where the focus is that "logistic support must be focused towards ensuring the success of the operation" and prescriptions of elements such as responsibility and authority. A logistic staff may be divided into sections based on branch or geographic area. Each section may in turn also be divided into tasks and roles. The size of the logistic staff can vary greatly, depending on the environment and complexity of operations. NATO in example work with a "Multinational Joint Logistic Centre", which exists outside of the force commander's staff, but runs as a separate entity/unit, with only a few logistic personnel in the commander's staff who act as liaisons.

Plans and strategy: The plans and strategy office is responsible for civil military operations (CMO) strategy planning. At the unit level, the S-5 is the primary adviser to the commander on the civilian-to-military and military-to-civilian impact of the mission/operation within the host nation's (HN) area of interest (AOI), area of operations (AO), or the target area of interest (TAOI). The G5 serves as the mission support office (MSO) at the division level and HHC for civil military plans and strategy.

Signal (communications and IT): The signal office directs all communications and is the point of contact for the issue of communications instructions and protocol during operations as well as for communications troubleshooting, issue, and preventative maintenance. Communications at this level is paired with digital as well as voice (radio, computer, etc.). At the unit level, S-6 is also usually responsible for all electronic systems within a unit to include computers, faxes, copy machines, and phone systems.

Training: The training branch will organize and coordinate training activity conducted by a Headquarters and also supervise and support subordinate units.

Finance: The finance branch, not to be confused with Administration from which it has split, sets the finance policy for the operation. Operationally, the Administration and Finance may be interlinked, but have separate reporting chains.

CIMIC: Civil-Military Co-operation: Civil-Military Co-operation or civil affairs are the activities that establish, maintain, influence, or exploit relations between the military forces, the government or non-government civilian organizations and authorities, and the civilian populace in a friendly, neutral, or hostile area of operations in order to facilitate military operations and consolidate and achieve mission objectives.

Commonwealth staff system: The "Commonwealth staff system", used by most Commonwealth nations, is largely based on the British military's staff system with nation-specific variations.

By country: 

Australia: Following Australia's Federation in 1901, the Australian Commonwealth Military Forces (now the Australian Army) adopted many of the practices of the British Army, including its staff system. While this approach was modified and adapted over the course of the 20th Century, the British three branch system and nomenclature remained a feature of Australian practice until 1997 when it adopted the Common Joint Staff System, based on the NATO or Continental/General Staff System, across all three services.  The primary reasons given for this were the ability to standardise staff organisations across the breadth and depth of the services, and; improve interoperability between America, Britain, Canada and Australia, as well as NATO partners that employed this system. At this time the Australian Defence Force also developed its own Joint Military Appreciation Process (JMAP), itself derived from the US Tactical Decision-Making Process and UK Individual Estimate.

Canada: The head of the Royal Canadian Navy, Commander of the Royal Canadian Navy, is also titled as Chief of Naval Staff.
The head of the Royal Canadian Air Force, Commander of the Royal Canadian Air Force, is also titled as Chief of Air Force Staff.
The head of the Canadian 
Army, Commander of the Canadian Army, is also titled as Chief of Army Staff.

United Kingdom: 

Army staff: The British Staff System was a product of the Esher Committee Report of 1904, which investigated the conduct of the late Victorian era British Army in the Second Anglo-Boer War and the Haldane Reforms from 1906-1912.  This staff system was captured in Field Service Regulations, Part II, Organisation and Administration, released in 1909 and later in the Staff Manual 1912. This system remained in use until 1984, when the United Kingdom began to use the Continental or NATO system. The British Staff System was based on the following:

Three branches:
G branch: The general branch, responsible for operations, intelligence and training.
A branch: The administration branch, responsible for all aspects of personnel management.
Q branch: The quartermaster branch, responsible for logistic and equipment support.
Positions: positions were labelled as follows, may also be styled GSO I, GSO II, GSO III:
GSO1, General Staff Officer (Grade 1): The chief of staff, ranked a lieutenant colonel or colonel. He was in charge of the general staff branch, responsible for training, intelligence, planning operations and directing the battle as it progressed. Most orders from the general officer commanding (GOC) were actually written up and signed by the GSO1.
GSO2, General Staff Officer (Grade 2): Ranked a major.
GSO3, General Staff Officer (Grade 3): Ranked a captain.
In the British system, staff are outranked by command officers. The staff cannot in theory (and largely in practice) say "no" to a subordinate unit; only the commander has that ability. This ensured a clear chain of command, and reinforced the idea that staff do not command, but exercise control on behalf of their commander. By contrast, in the American system, commanders are frequently outranked by staff officers. For example, within a battalion, the S-3 is a major while company commanders are captains. The principal staff officers at any HQ were always outranked by the subordinate commanders:

Lieutenant colonels commanding battalions or units in a brigade outrank the brigade major and the deputy assistant adjutant and quartermaster general
Brigadiers commanding brigades in a division outrank the colonel GS and colonel AQ
Major generals commanding divisions outrank the brigadier GS and assistant adjutant general and assistant quartermaster general at a corps HQ

Brigade level: Branches as brigade were as follows. A and Q branches might be combined under a deputy assistant adjutant and quartermaster general, rank major (DAA&QMG).

G branch (operations) plans and executes operations.  The senior staff officer in brigade HQ held the appointment of brigade major (BM) with rank of captain or major, who coordinated the HQ. While the BM was responsible for the entire HQ, he concentrated mainly on "G" operational matters. A deputy BM GSO III generally looked after non-operational matters. Under the BM were several GSO III (rank captain) officers:
Operations (the senior captain)
Intelligence
Liaison. The Liaison section often had several lieutenants attached from the brigade's combat units.
Air
A branch:  It handled all personnel matters such as awards, postings, promotions, medical, chaplains, military police and so forth. There were usually one or two GSO III officers in A branch.