An Armored Fighting Vehicle (AFV) is a military ground vehicle that's plated with any significant amount of armor, and is armed for combat. Even the lightest AFVs are difficult if not impossible to defeat with small arms alone. Their primary purpose is to provide fire-support to the Infantry, but most can also load and transport infantry from place to place.
The smallest AFVs are wheeled armored cars, fast but vulnerable. The largest are the tanks, tracked vehicles carrying enough armor to shrug off most attacks. Therefore, the collective term "Armored Fighting Vehicle" is very broad and largely technical. It is rarely if ever used but helps categorizing here.
AFVs appear in almost every mission, although they are far more common as enemy-controlled threats than player-controlled assets. Some varieties of AFVs are significantly more common than others, particularly APCs. The ever-present threat of AFVs showing up is the reason why players are heavily laden with Anti-Tank Launchers during most missions - as these are practically required to defeat an AFV.
Specifications
The term "Armored Fighting Vehicle" pretty much explains itself: It refers to any vehicle that is armored and ready to fight. This stipulates an amount of armor that is at the very least "significant" - capable of reliably withstanding Small Arms fire from enemy infantry and light vehicles. Vehicles fitted with just enough armor to make small arms less effective against them are not classified as AFVs.
Beyond this basic definition, AFVs can have wildly different attributes based on their particular class and designed purpose. They are generally split into several sub-categories that more narrowly define their capabilities and combat use.
Armored Cars
An armored car is a wheeled AFV with limited armor and armament. Although armored enough to reliably withstand most small arms, its exposed systems (primarily its wheels) and glass windows provide a vulnerability for the enemy to exploit. Additionally, particularly heavy small arms as well as other weapons available to Infantry are more easily capable of defeating it.
On the other hand, an armored car is a relatively small and fast target. It uses these features to quickly engage and disengage enemies without exposing itself to danger for long periods of time, for fear of retaliatory attacks.
An armored car is typically fitted with one or more machine guns as its primary weapon. It is therefore a capable anti-infantry platform, but rarely useful against other AFVs. Armored cars are typically capable of carrying roughly a fireteam of infantry inside the armored hull.
OPFOR is generally more fond of using armored cars, including the Vodnik and BRDM (although the former comes closer to be classified as an APC). BLUFOR instead fields a wider variety of recon vehicles, but does offer a couple of rarely-seen armored cars like the HMMWV
Armored Personnel Carriers
An armored personnel carrier is a wheeled or tracked AFV with substantial armor and a large carrying capacity, coupled with a heavy anti-infantry weapon or other similar weapon system. APCs are better armored than armored cars, with fewer vulnerabilities to small arms fire and slightly better effectiveness against the enemy. On the other hand, they are somewhat slower, heavier, and make an easier target for enemy fire.
APCs are used to ferry troops into and out of combat, and may also ferry troops between objectives. Once the troops are dismounted, however, the APC turns into a fire-support weapon that can adequately defend the infantry against enemy infantry as well as vehicular threats up to and including other APCs.
APCs are typically armed with a heavy machine gun. Some models are instead fitted with heavier weapons, particularly autocannons, to increase their firepower substantially. In that case, that particular variant may be considered an IFV, although still retaining the weaker armor and higher mobility of the APCs model.
APCs are some of the most common AFVs on the battlefield, and are most commonly seen as AI-controlled enemy vehicles rather than player-controlled. BTRs of various models are the most common, followed by M113s. There are several other APC models for different armies.
Infantry Fighting Vehicles
An Infantry Fighting Vehicle is a well-armored ground vehicle - typically tracked - that carries one or more relatively-powerful weapons. Although still capable of carrying some infantry like an APC, the IFV is a stand-alone weapon platform that is normally able to fight on its own and defend itself from threats even without infantry to support it.
IFVs are not as well-armored as tanks, but their armor is easily capable of ignoring all small arms fire and even small explosives. The side and rear armor of an IFV is insufficient to stop HEAT warheads, but some models are fitted with Slats to mitigate this problem. No part of an IFV is particularly capable of stopping armor-piercing cannon shells, however, which means that tanks are still a major threat to this type of vehicle.
IFVs aren't typically relied on to transport infantry, even though they can do so if the situation requires. An IFV's primary role is to provide infantry with heavy fire-support against almost any type of target. Most (but not all) IFVs carry an autocannon that is effective against both infantry and enemy vehicles - up to and including other IFVs - and are very effective against low-flying enemy aircraft. The autocannon may be equipped with armor-piercing rounds to have some chance of defeating enemy tanks as well. In addition, several IFV models can fire anti-tank guided missiles to destroy enemy armor more effectively.
The primary IFV for BLUFOR is the M2 Bradley series, with the Marine Corps preferring the LAV-25 series. OPFOR relies on the BMP model, which is one of the most common armored vehicles usually seen. Several heavily-armed variants of armored personnel carriers, especially the BTR-80A and M163 Vulcan, may be considered IFVs.
Tanks
A tank is the heaviest of all AFVs, in terms of both its armor and its firepower. All tank models are tracked vehicles, and they all carry a cannon, which is capable of firing powerful shells capable of defeating pretty much any ground target. Tanks therefore pose the ultimate ground threat. Tank armor is exceptionally heavy, having at least some chance to stop any weapon short of direct aerial bomb or air-to-ground missile hits. It even stands a chance of stopping enemy tank shells, which are typically deadly to any other vehicle on the battlefield. The side and rear armor of a tank are slightly weaker than the front armor, but are still formidable. A tank may sustain multiple hits from so-called "anti-tank weapons" before it finally succumbs.
The primary weapon of the tank is a cannon, which is typically loaded with armor-piercing shells for defeating enemy armored targets. Nevertheless, all tanks are equipped with heavy machine guns and/or high explosive rounds to tackle lighter threats without wasting those precious shells. Overall, a tank is quite capable of fighting on its own against a wide variety of enemy targets, and can indeed defeat almost any target if it gets the element of surprise. Several tanks working in unison can be very difficult to stop.
Enemy tanks are not a terribly uncommon sight on the battlefield, but most often appear as AI-controlled enemies. Player-controlled tanks aren't very rare either, but significantly less common. Otherwise, friendly tanks are typically used as infantry support, in a manner very similar to IFVs. Nevertheless, most tank models are completely incapable of carrying troops inside their hull (though depending on model and game version, troops may be able to ride on top of the tank's hull).
The most common tank in Arma is the Russian T-72, which is also the most common tank in the world right now. The primary American tank model is the M1A1 Abrams and its variants. Other tanks seen include the T-34, T-55, the German Leopard, the British Challenger, and the Israeli Merkava.
AFV Crews
Armored Fighting Vehicles require at least two crew members, but most have three. There are some vehicles with slots for a fourth crew member, such as the M1A1 Abrams, but this provides very little in terms of in-game benefits. Some mods allow AFVs to be one-manned as well. The primary crew member is the vehicle commander, who has the authority to decide what the vehicle would do at any given point, with regards to the guidelines set by whoever his superior while also taking into account the vehicle's current situation.
The second-in-command is the vehicle gunner, who mans and operates the vehicle's primary weapon. His primary task is to aim and fire the gun as directed by the vehicle commander, and at all over times to observe the area to the best of his ability.
Finally, there's the driver, who is responsible for moving the vehicle itself. The driver's job is the least involved, but they must stay alert at all times both for enemies within their (limited) view and to be ready for movement orders.
In case the vehicle only requires two crewmembers, or in case one is dead or missing, the remaining crew members can take on the additional responsibility to make up for this. For the most part, an AFV Commander will take over the gunner's position and vice versa, leaving the driver to do his job. It is much rarer to see AFV commanders or gunners driving the vehicle, as it significantly reduces their ability to remain aware of the vehicle's situation from the driver's seat. This is in contrast to light vehicles, where the commander usually drives the vehicle himself by default.
Organization
AFVs rarely make up the backbone of the force. Lighter AFVs (Armored Cars and APCs) are support vehicles, meant to fight alongside Infantry who make up the backbone themselves. Tanks and IFVs typically perform the same infantry-support function, but on very rare occasions may actually be the primary units in the force (typically with little or no infantry presence whatsoever).
Mechanized Infantry
An infantry company or infantry platoon supported by AFVs is called a mechanized company or mechanized platoon, respectively. It is organized similarly to a regular infantry force, with the exception of the AFVs themselves, who are added in one way or another to the regular infantry structure.
There are two primary ways in which this can be done:
Close Support: In this setup, each AFV or Ppair of AFVs is attached to a specific squad. It is tasked with supporting that squad, and will follow it around at all times. It may be required to transport that squad, if possible and necessary. A Close Support AFV typically switches its radio over to the squad's unique channel so that it can coordinate directly with the squad leader and occasionally receive orders from him. An AFV or AFV Section in this role typically receives a callsign based on the callsign of the squad to which it is attached, e.g. "Bravo Victor" for a vehicle attached to Bravo squad. This setup is more common with Armored Cars and APCs, which are designed for this specific role.
General Support: In this setup, AFVs are typically grouped together into section-sized or platoon-sized units (2 or 4 vehicles, respectively). The unit is commanded by the vehicle commander of whichever vehicle receives the first slot in the unit, and then is subordinated directly to the infantry company commander or platoon leader. Being separate from the infantry's lower command structure allows the unit to be moved around more freely, to provide fire support to whichever infantry squad requires it at the time. This flexibility provides more generalized and mobile protection for the infantry, useful when operating over a large, open area. This type of setup is more suitable for IFVs and especially tanks.
In both cases, ultimate authority rests with the force's top commander. Nevertheless, each vehicle commander retains a significant autonomy to decide what his vehicle should do at any given time, as only the vehicle commander fully understands his vehicle's current situation. Of course, a vehicle commander must keep his superior's standing orders in mind at all times.
Armor Company / Battalion
On rarer occasions, heavier AFVs (Tanks and sometimes IFVs) operate with little to no infantry support whatsoever. The AFVs are grouped into 4-vehicle units called Tank Platoons, which are all subordinated to an Armored Company. An Armored Battalion command may also be in place to coordinate the company's actions with Air Support and/or Artillery assets.