In the winter, the heated refrigerant from the compressor passes through an A-coil (pictured right---and they call it that because it is shaped like a capital "A"...this one is from a Hydron Module brand Geothermal unit).

Your blower motor from the forced air portion of your furnace forces air over this hot A-coil, warming that air which then flows throughout your home. In the summer, with a reversing valve, the refrigerant is chilled which makes the A-coil cold (like the inside of your refrigerator or freezer). The air passing over the A-Coil is then cooled and that chilled air is carried throughout your home.

In a radiant floor application, the inside heat exchanger absorbs the heat from the compressor and transfers it into the water that flows through your floor.