Military formation (ground)

A formation in military terms is a grouping of units.

Brigade

A brigade is the smallest type of formation, and generally consists of two or more battalions or regiments.

Division

Military divisions date back to the Napoleonic era. By the time of World War I, a division generally consisted of two or more regiments/brigades and numbered between 10 and 20,000 soldiers.

Corps

A corps is a grouping of divisions; usage of this term dates back to before the American Civil War.

Army

An Army is a grouping of, usually, two or more corps. Armies may also be responsible for the zone of communications behind a major front. They can also be repositories for "army-level" assets, such as super-heavy artillery, heavy armor, or other units of a specialized type not appropriately controlled by lower level headquarters.

An Army is generally commanded by a General or equivalent.