A heat pump is like a refrigerator or an air conditioner: it moves heat from a cold location to a warm location using a compressor that manipulates pressures in a refrigeration loop.
An air conditioner is designed to cool an occupied space. A refrigerator is designed to cool food. A heat pump is design to heat water or or an occupied space. But all such equipment cools one substance as it transfers heat to another substance. The refrigerator warms the kitchen; an air conditioner warms the outside air. Ideally both the heating and cooling being done simultaneously could both be used, and sometimes that is the case, but it is rare.
A heat pump used for space heat normally includes a reversing valve
so that it can heat the space in cold weather and cool the space in hot
weather. Almost all heat pumps manufactured to heat living spaces are therefore also air conditioners. A "reversing valve" allows it to be used to move heat in either direction: into or out of the living space.
Heat pumps move heat from cold to hot areas--the opposite of the normal direction--by manipulating the pressure of a refrigerant that evaporates at cold temperatures and ambient pressure. The refrigerant then condenses at the higher temperature because it's been compressed by an electric compressor to a high enough pressure that it is in a liquid state even at the higher temperature. (Exception: CO2 heat pumps are more complex than this.) The heat pump can thus be described as a tool to move heat from cold to hot areas via latent energy transfer induced by artificially manipulating the pressure of the working fluid, which is called a refrigerant.
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