Think of exactly a normal A/C in an automobile. Now, take the gas engine out of picture - it was turning the A/C pump via a belt. Replace that belt and engine with an electric motor. That's your A/C. Add a small radiator up front to act as the heat exchanger.
Add a valve with a selenoid to control it, make the freon go in reverse, it's now a heat pump.
Basically, and A/C IS a heat pump, it's just moving the heat from inside to outside. Heat pumps just let you run the cycle in reverse, so you move the heat from outside to inside.
The biggest problem with a heat pump is that at low temperatures, there is less heat available in the ambient air, so they need an auxiliary source, either electric or waste heat from something else.
Does that clear it up?