Classic military unit groupings—from ancient phalanx or hoplite formations to modern armored cavalry regiments—differ greatly in their specialization and size, and from country to country. With the United States military in the midst of a major reorganization—transformation, in the Pentagon’s lingo—here is a basic look at how modern ground, naval, and air units break down in terms of size and tasks.

• Squad. A small military unit consisting of ten to eleven soldiers, normally led by a staff sergeant.
• Platoon. A platoon is four squads: generally three rifle squads and one weapons squad, normally armed with machine guns and anti-tank weapons. Lieutenants lead most platoons, and the second-in-command is generally a sergeant first class.
• Company. Company-sized units, 130 to 150 soldiers, are normally commanded by captains. They consist of four platoons, usually of the same type, a headquarters unit, and some logistical capabilities. Companies are the basic elements of all battalions. In the artillery corps, a company would be called a battery. Cavalry units refer to this unit level as a troop.
• Battalion. A battalion, usually about 400-strong, is comprised of three rifle companies, a combat support company, and a headquarters company. Battalions often blend companies with different fighting specialties to take on tasks no existing unit is properly configured to tackle. Battalions normally fight enemy forces they can see and engage. This is defined as an area extending from less than 100 yards in forests, urban areas, and other close terrain out to about two to three miles from the battalion’s direct and indirect weapons-fire.
• Regiment. Formerly a major organizational unit, the regiment was eliminated from the force structure of the U.S. Army in 1957 (the Armored Cavalry Regiment is one of several exceptions). The U.S. Marine Corps, however, does still operate regimental units, comprised of five battalions—about 2,000-strong. The British Army and many modeled on it continue to use regiments, which are the largest of its organizational designations. A British regiment generally comprises several battalions (and, roughly, is akin to an American brigade).
• Brigade. Traditionally, the brigade provides mobility, counter-mobility and survivability, topographic engineering, and general engineering support to the largest unit—the corps—and augments the corps’ various divisions. Brigades can range from 3,000 to 5,000 troops, generally three-plus battalions, led by a colonel. Beginning in the late 1990s, the U.S. Army began formulating plans to shift the organizational foundation of its ground forces from large heavy armored and infantry divisions to “Brigade Combat Teams,” which are easier to airlift and regarded as more adaptable to the many forms of combat the modern military is likely to face. This transformation is taking place in stages. While the U.S. military searches for a permanent replacement for the seventy-ton M1A3 Abrams main battle tank, these Brigade Combat Teams are centered around the Stryker vehicle, a medium-weight armored vehicle with wheels rather than tracks. Ultimately, the Army envisions all its units “slimming down” both in size and in terms of portability to Brigade Combat Team levels. The Center for Strategic and Budgetary Assessments recently released an extensive study (PDF) of the Army’s transformation efforts.
• Division. Divisions perform major tactical operations for the corps and can conduct sustained battles and engagements. One division is made up of at least three maneuver brigades with between 10,000 and 20,000 soldiers, depending on the national army involved. American divisions are normally commanded by major generals and tend to be on the lighter side of the headcount. Divisions can have specialties such as light infantry, armored or mechanized infantry, airborne, and air assault. In other countries, division sizes vary widely. The British division, commanded by a major-general, generally is comprised of three brigades with a mix of infantry, armor, or armored cavalry. Russian (formerly Soviet) force structure generally favored much larger formations—12,000 plus, and in China’s military, up to 25,000. Wikipedia offers a good overview of foreign structures.
• Corps. The corps is the largest tactical unit in the U.S. Army. The corps is responsible for translating strategic objectives into tactical orders. It synchronizes tactical operations, including maneuvering, the firing of organic artillery, naval firing, supporting tactical air operations, and actions of their combat support, bringing together these operations on the battlefield. Each corps has between two and five divisions, and specialized brigades depending on the mission.

• Flotilla. A flotilla is any group of two or more ships.
• Squadron. This term is not widely used in the U.S. Navy with regard to ships with the exception of destroyer squadrons (DESRONs) and Amphibious ship squadrons (PHIBRONs), which are nominally commanded by captains. Otherwise, a U.S. Navy squadron generally refers to aviation—carrier or land-borne aircraft operated by the U.S. Navy or U.S. Marine Corps. Navy aviation squadrons are normally led by a commander. In foreign navies, however, a naval squadron often designates a small group of ships of the same type (submarines, for instance) or of various types tasked with a particular mission such as coastal patrol, blockade, or minesweeping.
• Task Force. A group of ships temporarily brought together for a specific mission, usually involving multiple ship types and centered around a capital ship—i.e., an aircraft carrier, amphibious assault ship, or other major naval combatant. Task Forces generally are commanded by an admiral and divided into Task Groups, each of which focuses on individual activities such as reconnaissance, anti-submarine warfare, logistics, or amphibious landings. Missions can be further narrowed into Task Units and Task Elements, if a Task Force commander so chooses.
• Strike Groups: These formations come in two basic types: the Carrier Strike Group (CSG) and the Expeditionary Strike Group (ESG), which would feature an amphibious assault ship at its center rather than a big deck carrier. CSGs generally are commanded by rear admirals, and ESGs by rear admirals or a Marine brigadier general. Both strike groups have destroyers, cruisers and a submarine attached as part of the force. Once in theater, these units can either operate in concert in supporting and supported roles or can be detached singly or in groups to perform other tasks and missions.
• Fleet. The largest formation inside any navy, fleets represent the assignment of particular ships and generally are regionally organized and commanded by an admiral. Large navies, like Russia ’s, may have separate fleets for far off regions. Russia’s navy includes a Northern Fleet, Black Sea Fleet, Pacific Fleet and several other smaller squadrons covering the Arctic, Mediterranean and even the Caspian Sea. The U.S. Navy is organized into five numbered fleets assigned in the Atlantic (Second Fleet, based in Norfolk, Va.), Third Fleet (Pacific Coast, based in San Diego, Calif.), Fifth Fleet (Persian Gulf, based at Manama, Bahrain), Mediterranean Sea (Sixth Fleet, based aboard the USS LaSalle, a command ship), and the Seventh Fleet, (based in the Pacific at Osaka, Japan). The First Fleet is an informal name for the U.S. Coast Guard. Other historic and defunct fleets are listed here by the Federation of American Scientists.
• Ship “Classes”. “Class” is an ambiguous term and its usage needs to be determined by the context of the sentence. At the highest level, classes of ships include major groupings like nuclear-powered Aircraft Carriers (CVNs), nuclear-powered Guided Missile Cruisers (CGNs), Guided Missile Destroyers (DDGs), Guided Missile Frigates (FFGs), Amphibious Assault Ships, (LHAs), sometimes called helicopter carriers, and a variety of submarines, including nuclear-powered attack subs (SSN) and huge, nuclear-powered missile subs or “boomers,” (SSBNs). The Navy’s website provides a full listing of active ship classes, as well as many historical classes, like Battleships (BB) and Escort Carriers (CVE), which no longer are in service. However, a ship’s “class” also can refer to the specific design it and its sister ships followed during construction. For instance, most of the U.S. Navy’s fleet of CVNs—nuclear-powered aircraft carriers—today are from the “Nimitz-class,” named for the first ship of its design, the USS Nimitz, which joined the fleet in the late 1970s. Similarly, an entirely new “class” of DDGs (destroyers) now is under construction. Designated DDG 1000 and named the “Zumwalt-class,” the first of these ships is not expected to join the fleet until sometime after 2010.
• Carrier Air Wing. The carrier-borne version of a U.S. Air Wing. Both the Navy and the Marine Corps operate fixed wing aircraft off of carriers. A U.S. Air Wing refers to three to foursquadrons of aircraft—approximately 72 aircraft total. Each squadron includes two or three “flights” of eight to twelve aircraft, led by flight commanders. Additionally, there are "Type Wings" focusing on training, equipping and maintaining of specific aircraft types. For instance, Helicopter Strike Maritime (HSM) Wings on the East and West coasts provide oversight to the HSM in their regions.
• Marine Air Ground Task Force (MAGTF). The MAGTF is the Marine Corps' basic organizational construct and is "task organized." These combined arms team are self-sustaining and equipped for joint/combined operations. The MAGTF is composed of a Command Element, Ground Combat Element, Aviation Combat Element and Combat Service Support Element. MAGTF's range in size from a MEU to a MEF (2,200-48,000). There also are so-called "Special Purpose MAGTF's" organized in response to specific missions such as the Tsunami relief in Indonesia or Hurricane Katrina support.
• Marine Expeditionary Unit (MEU). MEUs, often based on U.S. Navy Amphibious Assault Ships (LHAs), are a 2,200-strong landing force comprised of a reinforced infantry battalion, a composite helicopter squadron, a logistics group, and a Marine Special Operations company. They have a small compliment of tanks, light-armored vehicles and field artillery. They generally have some light-armored vehicles and field artillery, but lack the heavy tanks and guns of a similar sized army brigade. As ship-borne units, they are designed to be highly maneuverable and self-contained, including special operations, engineering, intelligence, and reconnaissance capabilities. The MEU generally generally is commanded by a colonel. MEUs usually are the first U.S. forces on the ground in combat environments and are designed to stabilize or hold an area until reinforcement arrives.
• Marine Expeditionary Brigade (MEB). A larger version of the MEU, with more armor, artillery, and air support, and built around a Marine regiment rather than a battalion (i.e., 2,000 rather than 400 Marines). These units specialize in a range of tasks, from “forced entry’ to larger humanitarian missions.
• Marine Expeditionary Force (MEF). The MEF, with up to 48,000 Marines, is the main combat formation in Marine Corps operations. The MEF is comprised of a division of U.S. Marines, including tanks and artillery, a Marine Air Wing with a strong ground support contingent, plus a Marine logistics group, all commanded by a lieutenant general. There currently are three MEFs, based in Asia (Okinawa), North Carolina (Camp LeJuene), and California (Camp Pendleton, Calif.). Since 2003, one has been deployed to Iraq at all times. A look at how the Marines envision using MEFs, MEBs, and MEUs can be found in the Corps’ “Force 21” plan (PDF).
• Float. This informal Marine Corps jargon refers to duty aboard any Navy vessel. All major U.S. Navy vessels include Marines as a landing force and boarding party. On nuclear-powered vessels, the Marine detachment also guards the reactor. Amphibious ships, of course, can carry large contingents of Marines for landing assaults, up to 2,200 in the case of the largest Amphibious Assault Ships (LHAs).

• Flight. The smallest organizational grouping of personnel, usually commanded by a captain. It is equivalent to an Army company, though Air Force flights often consist of far fewer people.  A the organizational flight is sometimes confused with a "flight" of aircraft, which is a tactical formation of two aircraft.
• Squadron. Literally a "square," in Latin, a squadron is the basic fighting organization of the Air Force.  Usually commanded by a lieutenant colonel, squadrons range in size from 30 to 500 personnel depending upon the mission.  Fighter squadrons typically are assigned 18-24 aircraft.
• Group. Three or more squadrons typically form a group. Groups are identified by one of four functional missions—operations group, maintenance group, mission support group and medical group—and are commanded by a colonel. In British usage, however, an Air Group is a much larger formation, roughly the equivalent of a U.S.tactical air force, such as U.S. Air Force Europe (USAFE) or Pacific Air Force (PACAF).
• Wing. An Air Force wing typically consists of four groups—one from each funcational specialty above. Typically commanded by a senior colonel or brigadier general, the wing commander is often the senior officer on a military installation.  Wings are named based upon a wide variety of missions—fighter wing, bomb wing, airlift wing, space wing, special operations wing, etc.
• “Numbered” Air Forces. Numbered Air Forces, or NAFs (prounounced naffs) consist of at least two, but often ten or more wings.  NAFs are Air Force equivalents to Army divisions, and are commanded by major generals.  Made famous in World War II (as in the mighty 8^th Air Force bomber force in England), NAFs were virtually eliminated from the Air Force structure in the 70s and 80s, but were reinstituted in the 90s. (Wikipedia includes a full listing of numbered air forces, plus the larger "major commands" they are subordinate to).
• Major Commands . The U.S. Air Force divides its forces into eight functional and overseas major commands (called "majcoms"), commanded by a four-star general and consisting of one or more NAFs and a large headquarters. The current Air Force active duty majcoms are: Air Combat Command (ACC), Air Education and Training Command (AETC), Air Force Material Command (AFMC), Air Force Space Command (AFSPC), Air Force Special Operations Command (AFSOC), Air Mobility Command (AMC), Pacific Air Forces (PACAF), and US Air Forces in Europe (USAFE)  regional and specialized commands—Strategic Air Command or SAC, based inNebraska , being perhaps the most famous. Others include Air Force Special Operations Command (AFSOC), Military Airlift Command (MAC), and a series of regional commands based around the world. A fuller list can be found on GlobalSecurity.org.