North Korea
BMP-1: About 200 of these Soviet vehicles are in the North Korean ranks, and they fall into the "infantry fighting vehicle" category. An IFV is an expansion of the APC concept in that it's well armed and armored and designed to fight alongside the troops after dropping them off, rather than having the minimum necessary for self-defense. In this case, the BMP-1 is armed with a 73mm semi-automatic cannon with 40 rounds, a 7.62x54mm coaxial machine gun, and a 9M14 Malyutka anti-tank guided missile launcher with 4 missiles. The missiles have most recently seen action in use with the rebels in the Libyan Civil War, and are probably the cheapest anti-tank missile you can get (on the level of under $1000 per unit). Disadvantages of the system include being manually guided to the target through a joystick and TV screen (which takes considerable training) and a low velocity that takes up to 30 seconds to fly to the maximum range of 3 kilometers, allowing far targets ample opportunity to flee or return fire. Along with the 3-man crew, the BMP-1 can carry 8 passengers and is fully amphibious for crossing rivers and lakes.
The main gun is meant to fire HEAT anti-tank rounds, which gives it good capability against light armor and it could probably at least work against the old Patton tanks of South Korea, but high-explosive rounds for anti-infantry use must be manually loaded. Its cyclic rate of fire is 8 to 10 rounds per minute, meaning around 6 seconds to load the next round. This is slower than the Bushmaster chaingun used on the American Bradley, but it's a larger projectile. The turret has no stabilization, which massively reduces accuracy when on the move. The gun also has a blind spot over the commander's hatch at the front-left of the vehicle, as it has to be raised over the infrared spotlight while rotating. One famous aspect of the BMP is that the rear doors are hollow fuel tanks, but this isn't as bad as it may seem: they're really only filled with fuel during relatively peaceful transport and are simply filled with sand or left empty when entering a combat zone.
VTT-323: This is the North Korean version of the Chinese Type 63 APC, a simple design comparable to cheap APCs of the time like the American M113. It's practically a box on wheels. It can carry 10 soldiers in the troop compartment and is armed with either a 14.5mm machine gun and a 7.62mm machine gun or a twin 14.5mm mount. Some versions also fit anti-tank guided missiles and man portable anti-air missiles like the SA-7. The North Koreans usually use such vehicles in mechanized battalions where they're fitted with 82mm mortars or 107mm multiple rocket launchers for artillery bombardment. Considering the terrain of Korea, this allows them to be hidden in high altitude areas for bombarding into the valleys. Another variant, named by Western observers the M-2010, is fitted with improved optics on an elongated chassis.
Type 63: The DPRK also has a number of original Type 63s. They're effectively the same as the VTT-323, but typically only armed with a single 12.7mm heavy machine gun for defense. It can still fit the mortars or rocket launchers of the VTT-323, but they both share a similar role when not armed for bombardment: get in, drop off the troops, and get out. The machine gun only provides limited defense against infantry and light vehicles like Humvees.
BTR: Various vehicles in the Soviet BTR series are in use, with most of the 1000+ number being the BTR-60. The BTR is basically an 8-wheeled metal box, armed with a 14.5mm heavy machine gun in a rotating turret and a coaxial 7.62mm machine gun. It also includes firing ports in the sides, allowing for soldiers to add to the defense by shooting their rifles or submachine guns from inside the armor. The BTR-60 can carry 14 to 16 troops depending on variant, who disembark from side or roof hatches (as the engine is in the rear). The armor is extremely thin, only meant to protect against small arms and shrapnel; a 7.62mm machine gun like the M240 or M60 could easily penetrate the side and rear armor under 100 meters, which makes it extremely vulnerable to ambushes or even just machine gunners who happen to get close. It's amphibious, but prone to breakdowns that leave it floating helplessly until the water jet could be restarted or someone could give it a push back to shore. The BTR-80 drops the troop complement down to 7, but some variants have 30mm autocannons in place of the 14.5mm machine gun and the gunner has an infrared night sight. The BTR-50 is an amphibious variant.
Ironically, despite ostensibly having a better understanding of the terrain, the BTR fared poorly in the mountains of Afghanistan against Mujahideen fighters who sat higher than the gun could elevate. They also found in both the Afghanistan invasion and the Sino-Soviet Border Conflict of 1969 that it was extremely vulnerable to RPGs.
Type 55: A Chinese copy of the BTR-40, the vehicle has nothing in common with the rest of the BTR series. It was the first mass produced Soviet APC, 1950 manufacture, and is essentially an armored truck with a pair of 7.62mm machine guns (and optionally a third) for defense. The Type 55 is more suited for recon than transport into a combat zone: it has no roof to protect its 6 passengers and can only be fitted with a tarpaulin for protection from the elements (which requires removing all machine guns), and .50 caliber machine guns can easily penetrate the armor. The tires are 100% unarmored and vulnerable even to infantry rifles. It was replaced by the Soviets for not fitting the modern battlefield in any sense, and would be better suited to police action than actual warfare.
BTR-152: This is what replaced the BTR-40, but it's frankly little different. In most senses it's just an elongated BTR-40 and maintains the same "armored truck" design, including vulnerability to heavy machine guns. At least later versions had an enclosed roof. The truck is typically armed with a single 7.62mm or 12.7mm machine gun, and can optionally fit another two 7.62mm guns. Its one advantage is the ability to transport 18 troops, about two whole squads, though they're not well defended against anything larger than a rifle. The truck is also notorious for poor reliability (even before North Korean logistical issues fit in) and the cross-country mobility of....well, a cargo truck.
South Korea
M113: This was the primary American armored vehicle of Vietnam and later conflicts before the Bradley came into vogue, and it remains in use worldwide due to its very simple "box on treads" design and ease of use. South Korea in particular maintains 400. One major advantage brought by the M113 is the aluminum armor, which is thick enough to defend against small arms fire but light enough to allow for the vehicle to be air transported. This is an important consideration, as the Korean DMZ is heavily mined and blocked by obstacles and air transport of vehicles over it will likely be a major factor in an early offensive into North Korea before airfields can be captured or the DMZ taken apart for land travel across the border. The US military still uses large numbers in rear echelon duties, such as battlefield ambulances and engineer and mortar carriers. It typically only carries an M2 Browning heavy machine gun for self-defense, and it isn't meant to engage in any kind of combat against anyone but small numbers of infantry. It can carry 11 passengers, a decent amount.
It's important to note that the M113 is NOT a fighting vehicle; it's meant to drop off troops and leave, or stay in the rear altogether. The South Vietnamese tried to modify them into amphibious light tanks, but they simply didn't have the armor or weaponry necessary even when fighting guerrillas. Indeed, even ones upgraded with slat armor and reactive armor will likely be very vulnerable to ambush should South Korean troops try to take them to the front lines.
K200: An indigenous design, numbering over 2300, and intended to act as more of a mainline IFV. The K200 can carry 9 passengers, which is a single South Korean Army squad, and has a .50 caliber machine gun and a coaxial 7.62x51mm machine gun in a rotating turret; the turret can also be fitted with a single 20mm Vulcan gun (a 6-barrel Gatling gun as used on fighter aircraft) and the rear can carry mortars, while a Metis-M anti-tank missile launcher can be fitted to the side of the turret.. The side armor is thick enough to protect against 12.7mm machine guns and the rear can handle 7.62mm, and it can handle anti-personnel mines (though obviously any kind of anti-tank mine will inevitably destroy it and everyone inside). It's able to operate amphibiously, allowing it to cross rivers and lakes.
BMP-3: The latest variant in the BMP series and the successor by two generations of the BMP-1 used by North Korea. The BMP-3 has a number of upgrades to bring it to modern standard, and fits a 100mm main gun with a coaxial 30mm autocannon and ANOTHER coaxial 7.62mm machine gun (as well as a pair of 7.62mm machine guns in the front of the hull and a 40mm grenade launcher) to allow for great levels of firepower. The cannon can also be fitted to launch anti-tank guided missiles with a range double that of the BMP-1's launcher. Competitive evaluations have demonstrated that unlike the BMP-1, the BMP-3 can effectively hit targets while on the move or while floating in the water. The BMP-3 can carry 7 soldiers and includes two additional seats apart from the main crew and troop complement. 70 of these vehicles are currently in South Korean ranks.
K532: This is the Republic of Korea designation for the Bandvagn 206, a Swedish articulated all-terrain carrier. It's difficult to classify due to its design, but it fits here better than anywhere else. The K532 consists of two units, both in the "box on treads" configuration: the front unit holds the driver and 5 passengers, while the rear trailer comes in a variety of configurations but often carries 11 soldiers in the troop transport capacity (for a total of 17 people including the driver). The vehicle is available in many different configurations, including being fitted with recoilless rifles and missile launchers for anti-tank defense. Even the USMC maintains some of them, and they're small and light enough to be transported by helicopter.
KM900: The South Korean designation of the Fiat 6614 APC, a four-wheeled amphibious carrier. It's expected to be phased out soon, but for the moment 400 remain. The vehicle carries 8 or 10 passengers depending on the variant and is armed with either a 20mm autocannon or a Browning M2 .50 BMG machine gun, as well as a coaxial 7.62mm machine gun. The hull is relatively thin welded steel, and it's not meant for defense against anything beyond small arms.
KAAV7A1: For Americans, this is the Assault Amphibious Vehicle, the premier amphibious landing craft of the United States Army. Like the USMC does, this is meant for South Korean Marines to make amphibious landings from offshore ships and is likely to see use from both armies in case of an invasion. It can carry an astounding 25 marines in full gear, and is armed with a Bushmaster 25mm chaingun or a Mk 19 automatic grenade launcher along with a Browning M2 heavy machine gun. Despite their advantages, the AAV-7A1 is notorious for poor protection and has even sustained minor damage from assault rifle and light machine gun fire. A number of them have been destroyed in the Middle East from RPGs, mortars, and tanks and they will be highly vulnerable to even the lightest armed tanks and APCs of the North Korean army.
K21: The newest IFV from South Korea, designed specifically for fighting other IFVs like the BMP series. 466 units are expected to be fielded, with the vehicles being introduced into service in 2008. The chassis is entirely fiberglass to reduce weight and increase road speed, and it can carry 9 passengers and is armed with a 40mm autocannon and two ATGM launchers. The vehicle was recently redesigned after two of them sank during amphibious operations due to technical problems. Because it's meant to go toe-to-toe with similar vehicles, the front armor is rated against 30mm armor piercing cannon rounds and even 152mm artillery shells landing up to 10 meters away.
United States
M2 Bradley: The Bradley is the primary IFV of the United States, and like the Abrams is considered the standard for its class. The Bradley carries 6 or 7 soldiers in its rear compartment and is armed with a 25mm chaingun, a 7.62mm coaxial machine gun, and a TOW anti-tank missile launcher (wire guided and controlled with a joystick by the gunner like the BMP-1). The 25mm gun has been found to be capable of killing T-55 and even T-72 tanks at close to medium range in the Middle East, allowing it to effectively fight the older North Korean designs. The TOW launcher will likely be able to score instant kills against all North Korean armor, but it can only be fired when the vehicle is stationary. Part of the design requirements was that the APC should keep up with the M1 Abrams tank, and as such it can maintain 41 miles per hour on the road or about half that off-road. While original vehicles could deploy a flotation curtain for amphibious operation, the armor upgrades have made it too heavy. The Bradley also has two rear firing ports in the rear doors fitted with M231 Firing Port Weapons, essentially modified fully automatic M16s with a high rate of fire and designed for extended suppressive fire. They can be removed for emergency use, but the heavy recoil and lack of a proper handguard or stock makes it a risky proposition.
The M3 Bradley is a separate vehicle, designed for reconnaissance operations: it replaces the troop compartment with a heavier ammunition load (including 1500 rounds for the chaingun and 12 missiles) and enhanced information and communication equipment, and has better NBC protection.
IAV Stryker: The Stryker is a big eight-wheeled APC best used for urban combat operations. It carries a variable number of passengers, usually around 6 to 9, and the frontal arc of the vehicle defends against 14.5mm heavy machine gun rounds while the rest can defend against 7.62mm rounds. They're often fitted with slat armor and explosive reactive armor for defense against RPGs, as well as a pressurized crew compartment for NBC protection and an automated fire suppression system. The Stryker is tough enough that they've been rolled by the force of an IED explosion and the crew survived (though the vehicle required a factory rebuild). The Stryker is typically armed with a remotely controlled turret with a .50 BMG machine gun or Mk 19 automatic grenade launcher and a coaxial 7.62mm machine gun, but the platform can also be fitted with mortars or even a 105mm light tank cannon (modified from the one on the M1 Abrams).
The United States and South Korea, as expected, maintain the most modern infantry transports of the trio. However, South Korea also has a good stock of older models that are somewhat comparable to the North Korean vehicles without the Soviet design features. The most numerous North Korean APC is the BTR-60, which has a number of problems when used in extended combat (mostly very thin armor) and ironically a difficulty fighting in mountainous terrain....like North Korea. The standard American vehicle, the M2 Bradley, is more numerous than North Korean vehicles while also having a primary weapon that's both fast firing and capable of taking on many models of DPRK tank. However, it should be noted that Allied vehicles are not invulnerable to tank cannons or missiles and RPGs (which can be easily hidden for ambush) and many of them are susceptible to IEDs and similar roadside explosives.