AC (Air Conditioning) is a system that cools and dehumidifies the air in a space. It is typically used to cool a single room or area, and is usually powered by electricity. HVAC (Heating, Ventilation, and Air Conditioning) is a system that provides heating, cooling, and ventilation to a space. It is typically used to control the temperature, humidity, and air quality of an entire building or area, and is usually powered by electricity or natural gas. The main difference between AC and HVAC is that AC is used to cool a single room or area, while HVAC is used to control the temperature, humidity, and air quality of an entire building or area.