What is the primary mission of each of the Army's helicopters?
Ugh, well...I didn't want to type this much but the dog is being quiet and I have a full cup of coffee, so here you go. I'll try to avoid personal prejudices in my descriptions.
UH-60A/L/M: Multirole utility helicopter. Used for primarily for the movement of troops and equipment in all areas of the battlefield and all threat environments. The aircraft has the capability to perform air assault (think insertion of troops in hostile territory), air movement, and slingload missions. A slingload is a load of eqipment suspended below the aircraft from an external cargo hook, as pictured above.
The UH-60A is also used in MEDEVAC missions, though it is finally being replaced by the HH-60M, which is a newer and more capable aircraft designed for the MEDEVAC mission.
CH-47D/F: Heavy-lift cargo helicopter. Primarily used for bulk movement of cargo and outsized items, such as bridge spans or sea-land containers. The CH-47 currently operates only at night in Iraq to reduce vulnerability. They have shined in Afghanistan, as they are the only aircraft with the engine power to perform at certain high altitudes encountered by NATO forces. The top photo shows an iconic image from the early Afghanistan war (purportedly a USMA grad at the controls, but that may be a rumor), of a CH-47D. The F model is the newest iteration, and is basically an entirely new aircraft made by stripping old CH-47Ds down the bulkheads and rebuilding them as F models. You can identify an F model by the baby-poo green paint, instead of the olive drab of the D model pictured in the top photo.
AH-64D: The Apache is the Army's primary heavy-attack platform. It is armed with a variety of weapons, from the entire range of HELLFIRE missiles, to 2.75" aerial rockets, to a 30mm articulated cannon. The 64D is known as the Apache Longbow because it features the Longbow Fire Control Radar, or FCR, which is the cheese-wheel-shaped item on the rotor mast. The AH-64D was originally designed as a deep attack and anti-armor aircraft, intended to /replace the AH-1 Cobra series. In the current conflicts, the 64D has assumed a new role of providing overwatch, security, and fires in support of ground maneuver elements, typically of platoon size and below. Apaches also fly armed escort for UH-60s and, in odd cases, fixed-wing aircraft on final approach. The lenses in the nose-turret are the PNVS (pilot night vision sysem) and the MTADS (modernized target acquistion & designation system). The systems allow the aircraft to fly and fight in less than ideal weather conditions, day or night.
OH-58D(R): The Kiowa Warrior is the small but beloved steed of the venerable scout aviator. The OH-58D is a single-engine dual-pilot armed reconnaissance platform. Originally conceived as an armed version of the OH-58A/C/D Kiowa prograrm, the Kiowa Warrior was fielded with Task Force 118 as part of Operation Prime Chance. It has since become a "big Army" aircraft. The OH-58D's primary mission is armed reconnaissance. In the current conflicts, the OH-58D has gained a reputation as the premiere airframe for close-in support of ground maneuver troops, usually company or platoon patrols or air assaults. The aircraft is generally flown without doors attached, except in the case of extreme adverse weather, giving the pilots full visib ility. The aircraft is armed with HELLFIRE missiles, 2.75" rockets, or the M-3P .50 cal heavy machine gun (which has replaced older .50 cal models). The large "ball" atop the rotor mast is the Mast-Mounted Sight, which features a daytime TV camera, a thermal imaging system, and a laser rangefinder/target designator.