If you go here:
http://www.foxnews.com/story/0,2933,74845,00.html, it's all spelled out for you. I chose the US Army as the example here, but there are links on the right that go into detail for each of the branches of service.
Army Formations
• Squad: The smallest groupings in the army structure, squads are made up of 8 to 11
soldiers and are normally led by a sergeant.
• Platoon: Usually consisting of two to four squads, platoons are typically led by a
lieutenant, with a senior sergeant second in command.
• Company (in the infantry), battery (in the artillery) or troop (in the cavalry): The company,
battery or troop is made up of three to five platoons and is typically commanded by a
captain. It usually has a first lieutenant as the second in command and a first sergeant as
the senior non-commissioned officer.
• Battalion: The primary combat maneuver element of the Army, the battalion or squadron
is composed of four to six companies and is commanded by a lieutenant colonel with a
sergeant major as the senior non-commissioned adviser. A major acts as the executive
officer and second in command. The battalion is tactically and administratively
self-sufficient and can conduct independent operations of a limited scope. An armored or
air cavalry unit of similar size to a battalion is called a squadron.
• Brigade: The brigade or regiment is made up of two to five battalions under the
command of a colonel with a sergeant major as the senior non-commissioned officer.
Armored calvary and ranger units of similar size to a brigade are called regiments, while
special forces units are known as groups.
• Division: Typically made up of three maneuver brigades, as well combat support
brigades, they are commanded by a major general. The division performs major tactical
operations for the corps and is capable of sustained operations.
• Corps: A corps is made of two or more divisions commanded by a lieutenant general. At
this level, national intelligence assets are analyzed and command and control of the
whole theater operations are coordinated.
• Army: Armies can be identified as theater armies, field armies, or army groups.