EERE: Energy Basics
Heat Pump Systems
Like a refrigerator, heat pumps use electricity to move heat from a cool space into a warm space, making the cool space cooler and the warm space warmer. Because they move heat rather than generate heat, heat pumps can provide up to four times the amount of energy they consume.
Air-Source Heat Pump 
Transfers heat between the inside of a building and the outside air.
Ductless Mini-Split Heat Pump
Ductless versions of air-source heat pumps.
Absorption Heat Pump
Uses heat as its energy source.
Geothermal Heat Pumps
Use the constant temperature of the earth as the exchange medium instead of the outside air temperature.