SQUAD: 9 - 10 soldiers
The squad is the smallest element in the Army organizational structure. The leader is a non-commissioned officer (usually a sergeant of a staff sergeant). The size is dependent on the function of the squad. Squads make up platoons.

PLATOON: 16 - 44 soldiers
A platoon consists of two or four squads. The leader is usually a lieutenant, while a staff sergeant or sergeant first class is second in command. Platoons make up companies.

COMPANY/BATTERY/TROOP: 62 - 190 soldiers
Typically three to five platoons form a company. A company is normally commanded by a captain, with a first sergeant serving as the company commander's principal non-commissioned assistant. An artillery unit of equivalent size is called a battery. An armored or air cavalry unit of equivalent size is called a troop.

The company is the basic tactical element of the battalion.

BATTALLION/SQUADRON: 300 -1000 soldiers
A battalion is composed of four to six companies. A battalion is normally commanded by a lieutenant colonel, with a command sergeant major serving as the battalion commander's principal non-commissioned assistant.

The commander has a staff who typically oversees the battalion's mission, training, administration and logistics functions.

A battalion is capable of independent operations of limited duration and scope. There are combat, combat support, and combat service support battalions.

An armored or air cavalry unit of equivalent size is called a squadron.

Battalions/squadrons make up brigades/regiments.

BRIGADE/REGIMENT/GROUP: 3,000 - 5,000 soldiers
A brigade headquarters commands the tactical operations of two to five combat battalions. A brigade is normally commanded by a colonel, with a command sergeant major serving as the senior, non-commissioned officer.

Brigades may be employed on independent or semi-independent operations.

Armored cavalry, Ranger, and special forces units this size are categorized as regiments or groups.

There are normally three brigades in a division.

DIVISION: 10,000 - 15,000 soldiers
Divisions are numbered and assigned missions based on structure. A division, for example, may be infantry, airborne, air assault, mechanized infantry or armored.

Divisions perform major tactical operations for a corps and can conduct sustained battles and engagements. A division is normally commanded by a major general.

Two or more divisions constitute a corps.

CORPS: 20,000 - 45,000 soldiers
A corps is the deployable level of command required to synchronize and sustain combat operations.

Corps staff elements perform analyses of national intelligence asset information.

The corps provides a framework for multi-national operations. The corps is commanded by a lieutenant general.

The corps provides command, control and logistical support of two to five combat divisions.

The corps is the primary command and control headquarters for the conduct of battle within a theater of operations.

Source: From the Department of Defense and the Federation of American Scientists