The car's air conditioning system conditions the air, which means whilst cooling the air in the car down, it also helps to ensure there isn't too much moisture in the car, so it stops the air getting too humid. The air conditioning unit also cleans the air in the car by filtering the dust and other particles from the air inside the car.
The aircon unit is where you control the settings, set the desired temperature and can find the aircon switch to turn the car's air conditioning unit on. All of the components that make up the car's air conditioning unit are connected to the aircon unit.
If the aircon unit stops working completely or repeatedly switches itself on and off, for example, it will need to be looked at by a professional and a replacement aircon unit may need to be purchased for your car.