An air handler circulates air in a HVAC system. It is typically a metal-cased unit that houses various components, such as a blower and heating element, and it is connected to the duct work of a building to distribute air.
Air handlers control the temperature of a building and the humidity levels by using heat exchanger coils. The blower is typically an electric motor that controls the air flow rate of the handler. Large handlers sometimes contain multiple motors and are typically used in commercial buildings. Terminal units are small air handlers that commonly only contain an air filter, blower and coil and are typically used in residential settings.