An air source heat pump (ASHP) takes heat from the air outside and transfers it to water to heat the home. The process is exactly that used in a domestic fridge except that the heat is used to warm water for a heating system of either wet underfloor heating or radiators. There are several factors which affect the efficiency of an air source heat pump but one of the biggest is the outside air temperature so they may not be suitable in areas which experience sub-zero winter temperatures regularly.
The maximum efficiency presently available for an ASHP is around 300% so they are not as efficient as a ground source heat pump although they do benefit from lower initial cost and ease of installation.