Heat pumps are electric powered outdoor units that have the capability of cooling and heating. The only difference between a heat pump and air conditioner is the fact that a heat pump can provide heat.
During the summer months, a heat pump cools indoor air with R-410A refrigerant exactly like an air conditioner does: refrigerant is cooled and sent inside to the evaporator coil or fan coil (air handler) where it extracts heat from the indoor air. In the winter this process is reversed by extracting heat from the air outside. This heat is absorbed into the refrigerant and sent to the indoor coil. Even though air may be cold in the winter, it still contains heat.
Heat pumps are typically more efficient than a gas furnace when winter lows are 35-40° F. They are not ideal for northern winter climates where temperatures are well below freezing. Heat pumps have what is called a "balance point" which is a term used to describe the point where outside air temperature and a heat pump's capability match. Below this point, supplemental heat is required because the heat pump cannot pull enough heat from the air outside to adequately heat your home.
Hybrid systems are made up of a heat pump and gas furnace. The reason for a hybrid system is increased energy efficiency during the winter months. Heat pumps are usually more efficient than a gas furnace when the temperature is 40° F and above (this varies depending on local utility and fuel prices). Below 40° F a gas furnace will often be more efficient. In a hybrid system design, the heat pump and gas furnace automatically switch on and off depending on temperature, providing highly efficient heating.