Military Helicopters are primarily used by the Navy and United States Marine Corps to transport troops into a Combat Zone or LZ which is known as a Landing Zone. Sometimes they are known as Hot LZ's, which means a Landing Zone that is heavily engaged in Combat. Sometimes these are used in Combat Search and Rescue as well as Medical Evacuation. Usually NCO's or Non Commissioned Officers use their ground radios to communicate with the Helicopter Pilot to give them their ground location.
Military Attack Helicopters
These machines are armed and are usually used to take out enemy Tanks as well as used in Close Air Support. In the role of CAS, A Marine Corps Forward Observer, which can be a Marine with an MOS (Military Occupational Specialty) of 0861, is used to call in Grid Coordinates of an enemy bunker, a tank, a moving convoy or other enemy target so the Helicopter Pilot in the air will know exactly where to aim their missiles.
Grid Coordinates have to be estimated and adjusted sometimes for moving targets, based on the speed and direction of the moving target. Also note that the US Army has Forward Observers as well.
The word Helo is another popular term used by Troops.
Military Transport Helicopters
These are used to place troops in certain combat positions and locations. They are used as a support transport system for combat operations as well as being constantly used in training. Sometimes troops can exit the aircraft by Fast Roping, which troops are attached to a very heavy rope with a D-Ring.
Sometimes they exit on Parachute, as in the case of 1st Anglico Troops, the 82nd Airborne and some other elite units. And in some cases they are taken right to the ground and just jump off one of the sides.
Military Observation Helicopters
The purpose of these are to spy on enemy troops and to pickup intel and learn about their tactics. At first they use to use balloons. Later they started using lighter airplanes. The ability for these to stay in one location for a good period of time made them ideal for these missions. Two common Recon Helos were the Taylorcraft L-2 and the Fieseler Fi 156. The line of observation used to be limited to what the aircrew could see. Later on however, Optical Sensor Systems and Infrared Cameras substantially increased this range.