This is a very oversimplified answer, which can probably be better answered by an air conditioning professional. But for a simple example / understanding, imagine using an ORDINARY window air conditioner for BOTH summer cooling and winter heating. Basically a window air conditioner takes heat out of the air on one side, and blows HOT air out the other side. That is what it does in hot weather, takes heat out of the indoor air, and moves it outside. IF you were to REVERSE / ROTATE the air conditioner the OPPOSITE direction, it would take heat out of the OUTSIDE AIR, and add it to the INSIDE air. Wait, I can hear what you're thinking, but YES, there is SOME heat even in cold outside air. A heat pump is not very efficient in VERY COLD climates, so the manufacturers usually add an electric heating element to "boost" heat output when there is not enough available in the outside air. Now the heat pump, whether it be a window unit, or a fixed central heat and air unit, is basically an air conditioner which you DO NOT HAVE TO TURN AROUND to change the output. That is accomplished with controls for VALVING which can REVERSE the flow of the referigerant, depending whether heating or cooling is required! Also, for very cold regions, instead of having an air "radiator" [coil and fins] to collect the very little heat from the very cold outside air, some heat pumps use a buried [underground in the yard] tubing system [circulating a liquid similar to water] to collect heat from the earth. The heat collecting grid is buried deep enough to be where the earth stays relatively warm [compared to the surface and air above]. Also, in very hot summer regions, this area stays relative cool compared to the air, and can be used to "dump" the heat from the air conditioning mode, increasing efficiency, and reducing operational costs! VERY simply, a heat pump is a REVERSABLE air conditioner. j3h.