Nothing really, the only thing your doing is reversing the refer flow. The warm air that an a/c blows out in the summer is reversed and blows inside. And because the outside temp is cold you need to add defrost control so the unit doesn't become a solid block of ice.
This is what I tell HO that ask. It is a very simple explanation and covers most of the bases.
And if they ask I tell them the unit is picking up heat from the outdoor air and moving it inside the house, and the reverse is true in the summer.
A heat pump will change the direction of the refrigerant flow. That way insted of taking heat from your house and transferring it outside "like a staight a/c", it will absorb heat from the outdoor air and distribute it into your home.
Well I think if you look at the basic concept of airconditioning being "to remove heat from a place where it is unwanted and rejecting it to a place where it is not objectionable" can be reversed when addressing a heat pump to sat remove heat from a place where it is not objectionable and rejecting to a place where it is wanted.
Bottom line is an air conditioner doesnt produce cool, it moves heat from the evaporator to the condenser. A heat pump moves heat fromm the evaporator to the condenser also, it just switches which coil fills what role.
You also have to understand that there is no such thing as cold, only heat or a lack there of. Since the total absence of heat is absolute zero (not 0*F or 0*C). Anything above it contains heat, the HP or A/C simply moves it.