What is the difference between a 'Regiment', a 'Corps', a 'Battalion' and a 'Division'?

The Indian Army serves as the ultimate instrument for maintaining the unity and the integrity of the nation in the face of external threats and internal unrest and disturbances. The major tasks of the Indian Army are as follows:

1. Effectively project deterrence and dissuasion through the medium of strong, well-structured combat capability.
2. Be prepared to engage in and conduct all types of military operations, singly or jointly, in the entire spectrum of conflict.

3. Provide the requisite land forces component of the Strategic Forces Command.
4. Provide aid to civil authority when called upon to do so for maintenance of law and order, humanitarian aid and assistance during disasters and calamities or any other circumstances including maintenance of essential services.

5. Participate in United Nations peacekeeping operations in consonance with India’s commitment to the United Nations Charter.
6. Be prepared to render military assistance to friendly countries when required to do so.
Formation and Structure of Indian Army

Commands of the Indian Army :The Indian Army is divided into 6 operational commands and one training Command each headed by a Lt. General

1 Northern Command Udhampur

2 Western Command Chandigarh

3 Central Command Lucknow

4 Eastern Command Kolkata

5 Southern Command Pune

6 Western Southern Command Jaipur

7 Training Command Shimla

Corps: Next the line under the Tactical Area Commands are the Corps Headquarters, which are Field Army Headquarters elsewhere. It is headed by General Officer Commanding Corps in the rank of Lt General. The Indian Army’s combat formations are now grouped and tailored under many such Corps Headquarters (with some forces being retained under static Area Commands).

All Corps have numerical designations. The field force is grouped into Corps. Some of these are defensively oriented and have, over the years, acquired an unofficial – ‘Holding’. The others are called reserve or, unofficially again, ‘Strike’ Corps. The principal offensive formations of the Indian army are the three Strike Corps – 1 Corps, 2 Corps & 21 Corps.

These are built around a nucleus of a single armored division and two infantry divisions – probably with more mechanized brigades than basic infantry formations. Strike corps should be capable of being inserted into operational level battle, either as battle groups or as a whole, to capture or threaten strategic and operational objective(s) with a view to cause destruction of the enemy’s reserves and capture sizeable portions of territory.

Division: Each Division is headed by General Officer Commanding (GOC) in the rank of Major General. It usually consists of 15,000 combat troops and 8,000 support elements. Currently, the Indian Army has 37 Divisions including; 4 RAPID (Re-organized Army Plains Infantry Divisions) Action Divisions, 18 Infantry Divisions, 10 Mountain Divisions, 3 Armored Divisions and 2 Artillery Divisions. Each Division composes of several Brigades. A division can operate independently and is self contained.

Brigade: A Brigade generally consists of around 3,000 combat troops with supporting elements. An Infantry Brigade usually has 3 Infantry Battalions along with various Support Arms & Services. It is headed by a Brigadier. In addition to the Brigades in various Army Divisions, the Indian Army also has 5 Independent Armored Brigades, 15 Independent Artillery Brigades, 7 Independent Infantry Brigades, 1 Independent Parachute Brigade,3 Independent Air Defence Brigades, 2 Independent Air Defence Groups and 4 Independent Engineer Brigades. These Independent Brigades operate directly under the Corps Commander (GOC Corps). An Independent brigade can operate independently and is self contained.

Battalion: A Battalion is commanded by a Colonel and is the Infantry’s main fighting unit. It consists of more than 900 combat personnel.

Company: Headed by the LT Col or Major, a company comprises 120 soldiers.

Platoon: An intermediate between a company and section, a platoon is headed by a Captain or Lieutenant, or depending on the availability of commissioned officers, even a junior commissioned officer (Subedar). It has a total strength of about 32 troops.

Section: Smallest military outfit with a strength of 10 personnel. Commanded by a non-commissioned officer of the rank of Havildar or Sergeant.

It is explained by Manohar AM on Quora.