The Marine Corps is authorized by the Department of the Navy, which is spearheaded by the Secretary of the Navy. At the top of the organizational structure is the commissioned officer called the Commandant of the Marine Corps. This person is responsible for organizing, recruiting, and training so that they are ready for operation and command.
The U.S. Marine Corps is further organized into four subsections: the Headquarters Marine Corps, Operating Forces, the Supporting Establishment, and the Marine Forces Reserve. Those subsections then contain all or some of the following groups:
The United States Marine Corps dictates that any active fire team will include two gunmen, with a mnemonic ready team fire assist. In total, the fire team will normally consist of three Marines and a Corporal. The rifleman acts as a scout, the team leader is equipped with an M203 and works as the designated grenadier, a designated rifleman is equipped with the M249, and an assistant automatic rifleman is there to help with range finding and spot support.
Usually, this group consists of a rifle squad composed of several fire teams, each with four Marines, and a leader who is typically a Sergeant. These squads are sometimes broken down into other infantry squads, too, which may include: heavy machine gun, machine gun, mortars, anti-armor, assault weapon, and antitank missiles. These teams can be large, especially for Javelin Missile squads.
The platoons consist of 43 Marines who are led by a Sergeant. A weapons platoon usually has both a lieutenant and a gun sergeant due to the number of Marines. A rifle company weapons platoon will be equipped with a mortar section of 13 Marines, an assault section with 13 Marines and six rocket launchers divided into three distinct squads of two teams each, and then a medium machine gun section consisting of 22 Marines and six machine guns divided into three squads of two teams each. For the infantry battalion company, there will be three heavy weapons platoons that specialize in heavy machine gun, anti-armor, and mortar.
The Company and the Battery
This will consist of three or more platoons commanded by a single Captain.
Marine Corps Battalion
A battalion is a military unit that consists of hundreds of soldiers divided into a series of companies. Typically they are spearheaded by a Lieutenant. There are four companies to a battalion, and two or more battalions are maintained by a regiment. Normally, there will be four companies that will include three line companies and one headquarters and service company (H&S). Four battalions will include three lines and one H&S company. Keep in mind there are always variations to what is the standard breakdown of the organizational structure of the Marine Corps, especially when it comes to battalions.
Three regiments will make up a Marine Division. Four divisions equate to the Marine Corps. The leader of the Marine Corps reports directly to the Secretary of the Navy. The Marine Corps Expeditionary Forces consist of three groups, and each is formed by a ground combat, aviation, and logistics team.
The Aircraft Wings subdivision of the Marine Corps consists of squadrons that command flying and non-flying units of about two dozen aircraft, grouped into three or more squadrons or a complete regiment. The Aircraft Wings are equivalent to a division.
You can learn more about Marine Corps organization & find individual units here.