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This is the Marines organization. Read it, and remember it.

Fireteam: Consists of 3 Marines, led by a Corporal. (4 Including the Corporal)

Squad: 3 Fireteams. Led by a Sergeant. (13 Marines per Squad)

Platoon: 3 Squads. Led by a (1st or 2nd) Leuitenant. A Platoon Sergeant is the second in command of a platoon and a Navy Corpsman accompanies every platoon.

Company: 3 platoons per company. Led by a Captain. A weapons platoon is assigned to every company, consisting of a 60 mm mortar section, an assault section, and a medium machine gun section. There is also a company HQ, consisting of the following;

Company Commander
Executive Officer
First Sergeant
Gunnery Sergeant
Chief Clerk
Supply Sergeant
Administrative Personnel
Messenger
Two Radiomen [one for the Artillery FO]
Two Riflemen [Runners]

Battalion: A battalion consists of 3 companies (are you starting to see the trend now??)

Regiment: 3 battalions per regiment. (sometimes known as a brigade)

Division: consists of 3 regiments. Largest unit size in the Corps.

No, Its a regiment, and it consists of 3 battalions.

Slash.. unlike the Army or Air Force (which originates from the Army) the unit task organization is all based on the number 3.

Militarily speaking of course it is to allow for a 2 up, 1 down theory. You have the Primary, the Backup, and the Reserve.

Now same way

Fireteam
Squad
Platoon
Company
Battalion
Regiment
Brigade
Division
Corp

However the Marine Corp no longer has Brigades unless they task them out as MEB (Marine Expeditionary Brigades) which has not been done in forever.

The difference is in the level of controls and additions. technically a brigade was just the next level of infantry battalions without support elements. The division is the same thing as a brigade just with support elements. So because of the redundancy the USMC does not use the Brigade classifcation because we have no need for it unless we task out several battalions of infantry to perform some operation without the full need of division support. (e.g. Fleet Marine Forces stationed on ships.. hence the Expeditionary classification.) But usually they want support cause who wouldnt... and those are usually classifed as MEF's (or Marine Expeditionary Forces)

Additionally here is a link to a very handy website with TONS of information about the military (if you can sort thru it to find what you want)
globalsecurity.org

I hope that clarifies things for you all.

Semper Fi!