How they work
A reversing valve swaps the direction of refrigerant flow. In summer they cool your home like any AC; in winter they pull heat out of outdoor air (yes, even cold air has heat) and pump it inside.
Dual-fuel setups
Pair a heat pump with a gas furnace and an outdoor thermostat. Above the balance point, the heat pump runs (cheaper). Below it, the furnace takes over (more capacity). You get the best of both.