If you do not know exactly what an HVAC heat exchanger is, you might want to watch this video. There are many types of heat exchangers that are used. These boost efficiency and save space, as well as minimize the amount of refrigerant that is used.
It is meant to transfer thermal energy, and either a cool or hot fluid (which can be a liquid or a gas) is used. Thermal energy can be transferred using either conduction, convection, or radiation. Typically, in the case of an HVAC heat exchanger, either conduction or convection will be the method.
Conduction is a phenomenon that occurs when two materials of different temperatures are actually in physical contact with one another. Convection involves fluids moving and carrying thermal energy away. This can either occur via mechanical force or by a more natural method. Radiation involves the emission of electromagnetic waves from a surface. Usually, the fluids used in HVAC heat exchangers involve water, air, steam, refrigerant, or oil, as the medium used to transfer the heat. An HVAC exchanger will either heat or cool air or water. Most HVAC heat exchangers will come in either a plate or a coil design.