A heat pump is a type of heating unit that was very popular for a time, and is still used in some areas. It is basically an air conditioner in reverse.
An air conditioner is very complex in its chemicals, pressures, etc. But it basically works like this: It heats refrigerant material as it compresses, cools it down a bit, and them decompresses it making it cool. A heat pump does that same thing basically, but just does not do the cooling part. That is why most heat pumps are also AC units as well – they do basically the same thing.
So why do we not have very many heat pumps around here? First of all, they do not work well once the temperature hits below 0 degrees C. In fact, the lower the temperature the worse it works. Most people would have a hard time keeping their home warm. So what is it good for?
Emergency heat. Most of the time if you find one that is what it is for. It is a great thing actually. If your main heating unit goes down you can switch over to that unit. Even if it only keeps your house at just above freezing, it is still much better than frozen pipes.