Military rank

Military rank, or, more commonly, simply rank, is a system of grading seniority and command within armed forces or otherwise military-style organizations. The word 'rank' is the English corruption of the Farsi word 'rang' which means colour. Muslim armies regularly issued coloured flags to different elements of their armies allowing the average muslim soldier to be able to discern the 'rang' of a particular commander.
Within armed forces, the use of ranks is almost universal. The Chinese People's Liberation Army (1965–1988), the Albanian Army (1970–1991), and the Soviet Red Army (1918–1935) are rare examples of armed forces which abolished rank only to re-establish them after encountering operational difficulties of command and control.

Roman ranks
The use of formalized ranks came into widespread use with the Roman legions after the introduction of reforms by the consul Gaius Marius, which were completed around 60 BC. Under the new system, a legion would be commanded by a legate (legatus), typically a senator, for a three-year term. Immediately beneath the legate were six military tribunes (tribuni militum), five of whom were young men of Equestrian rank and one a nobleman who was headed for the Senate.
The fighting men in the legion were formed into ranks: rows of men who fought as a unit. Under the new system, legions were divided into ten cohorts (cohortes), each consisting of six centuries, each of between 60 and 160 men. Each century was led by a centurion (centurio), assisted by a number of junior officers. Centuries were further broken into ten contubernia, each of eight soldiers. Individual soldiers were referred to as soldiers (milites) or legionaries (legionarii).

Mongol ranks
The organization of the Mongol army was based on that of the Achaemenid Empire of Persia: the decimal system. The army was built upon a squad of ten (arban) led by an elected noncommissioned officer. Ten of these would then compose a company of a hundred (jaghun), also led by an elected noncommissioned officer. The next unit was a regiment of a thousand (minghan) led by an appointed Noyan. The largest organic unit was a ten thousand man unit (tuman) led by an appointed Noyan. Field armies would normally consist of three tumans with attendant siege units grouped into independent minghans under a unified command staff led by an Orlok. In addition, the quartermasters of the Mongol armies carried the rank of Yurtchi.

Modern ranks
All western military ranks descend from three sources. From renaissance mercenary titles come corporal (as though "corps man", but actually from French caporal, "chief"), sergeant ("servant"), captain ("head man"), and general (a shortening of the term "captain general"). From the era of the Napoleonic Wars came colonel (head of a column), lieutenant ("one who stands in lieu of"), major (a shortening of "adjunct major" or aide de camp), and marshall ("stable manager"). From World War II come a number of specialist ranks. Most modern military services recognize three broad categories of personnel. These are codified in the Geneva Conventions, which somewhat ambiguously distinguishes "officers", "non-commissioned officers" and "men".
Apart from possible conscripted personnel one can distinguish:

Commissioned officers
Officers are distinguished from other military members by holding a commission; they are trained as leaders and hold command positions.
Officers are further separated into three levels:

General, Flag, or Air officers
Officers who typically command units or formations that are expected to operate independently for extended periods of time (brigades and larger, or flotillas or squadrons of ships), are referred to variously as General Officers (army, marines, and some air forces), Flag Officers (navy), or Air Officers (some Commonwealth air forces).
General Officer ranks typically include General, Lieutenant General, Major General and Brigadier General, although there are many variations.
Flag Officer ranks, named after the traditional practice of showing the presence of such an officer with a flag on a ship and often land, typically include Admiral, Vice Admiral and Rear Admiral.
Air Officer ranks usually include Air Chief Marshal, Air Marshal, Air Vice-Marshal and Air Commodore.
In some forces, there may be one or more superior ranks to the common examples above that are given distinguishing titles, such as Field Marshal (many armies), Fleet Admiral (U.S. Navy) or Marshal of the Royal Air Force or other national air force. These ranks have often been discontinued, such as in Germany and Canada or limited to wartime and/or honorific promotion, such as in the United Kingdom and United States
In various countries, particularly the U.S., these may be referred to as "star ranks", for the number of stars worn on some rank insignia; typically one star for Brigadier General or equivalent with the addition of a star for each subsequent rank. While in the U.S. five stars has been the maximum used in all services excluding the U.S. Marines, the historical but still substantive ranks of General of the Armies and Admiral of the Navy are notionally referred to as "six star" ranks.
Some titles are not genuine ranks, but either functions assumed by generals, or honorific titles. For instance, in the French Army, Général de corps d'armée is a function assumed by some Généraux de division, and Maréchal de France which is a distinction denoting the most superior military office, but one that has often neutered the practical command powers of those on whom it is conferred.

Field or Senior officers
Field officers, also called "field-grade officers" or "senior officers", are senior officers who typically command units that can be expected to operate independently for short periods of time (battalions and regiments, large warships). Field officers also commonly fill staff positions.
The term "field(-grade) officer" is primarily used in by armies and marines; air forces and navies generally prefer the term "senior officer". The two terms are not necessarily synonymous.
Typical army and marine Field Officer ranks include Colonel, Lieutenant Colonel and Major. In many Commonwealth countries, the field rank of Brigadier is used, although it fills the position held by Brigadier General in other countries.
Typical naval Senior Officer ranks include Captain, Commander and Lieutenant Commander. In some countries, the more senior rank of Commodore is also used, a position that follows the flag flying tradition (above) of Flag Officers but through the use of a dove-tail pennant of rank instead of the flag or triangular pennant of other senior officers.
Commonwealth air force Senior Officer ranks include Group Captain, Wing Commander and Squadron Leader.

Company Grade or Junior Officers
The ranks of junior officers are the three or four lowest ranks of officers. Units under their command are generally not expected to operate independently for any significant length of time. Company grade officers also fill staff roles in some units.
Typical army Company Officer ranks include Captain and various grades of Lieutenant. Typical naval Junior Officer ranks include various grades of Lieutenant and/or Ensign. Commonwealth air force Junior Officer ranks usually include Flight Lieutenant, Flying Officer and Pilot Officer.

Warrant Officers
Warrant officers (as receiving authority by virtue of a warrant) are a hybrid rank treated slightly differently in each country and/or service. WOs may either be effectively senior non-commissioned officers or an entirely separate grade between commissioned and non-commissioned officers, usually held by specialist personnel.

Enlisted personnel
Enlisted personnel (called Other Ranks in some Commonwealth countries) are personnel below commissioned or warrant rank and make up the vast majority of military personnel.

Non-Commissioned Officers
Non-commissioned officers (NCOs) are enlisted personnel, under the command of an officer, granted delegated authority to supervise other military members or assigned significant administrative responsibilities. In U.S. Army parlance: "NCOs are the backbone of the Army!" NCOs are responsible for the care and direct control of junior military members.
Even the most senior NCO officially ranks beneath the most junior commissioned officer or warrant officer. However, most senior NCOs have more experience, possibly including combat, than junior officers. In some organizations, senior NCOs may have formal responsibility and informal respect beyond that of junior officers, but less than that of warrant officers. Many warrant officers come from the ranks of mid-career NCOs. In some countries warant ranks replace senior enlisted ranks.
NCO ranks typically include a varying number of grades of Sergeant and Corporal (air force, army and Marines), or Chief Petty Officer and Petty Officer (navy and coast guard). In many navies, the term rate is used instead of rank.

Other enlisted ranks
Personnel with no command authority usually bear titles such as Private, Marine, Airman/Aircraftman and Seaman. In some countries and services, personnel in different branches have different titles. These may have a variety of grades, but these usually only reflect variations in pay, not increased authority. These may or may not technically be ranks, depending on the country and/or service.

Appointment
Appointment refers to the instrument by virtue of which the person exercises their authority. Officers are appointed by a Royal Commission in most monarchies, or a Presidential Commission in many other countries. In the Commonwealth, Warrant officers hold a Royal or Presidential Warrant. NCOs are appointed by an instrument of appointment, a written document, often a certificate, usually from the service head. Entry into service is often referred to as enlistment throughout the English speaking world, even in countries where soldiers do not technically enlist.
Sometimes, personnel serve in an appointment which is higher than their actual rank. For instance, Commodore used to be an appointment of Captain in the Royal Navy, Lance Corporal used to be an appointment of Private in the British Army, and Master Corporal is an appointment of Corporal in the Canadian Forces

Information adopted from Wikipedia