Air source heat pumps absorb heat from the air outside which is then used to heat your radiators and hot water and also any underfloor heating systems in your home. It works very much like a fridge in that it extracts heat from its inside. Even if the temperature outside is -15C an air source heat pump can still get heat from the air. They work by delivering heat at lower temperatures over much longer periods. A unit is fixed to an outside wall and it will need plenty of space to enable the system to get a good flow of air. An ideal place for a unit would be a sunny wall outside.
Ground source heat pumps are buried the garden and extract heat from the ground through pipes. The heat is then used to heat your radiators, hot water, underfloor heating or a warm air heating system in your home. The pipes are laid out in a loop and are filled with a mixture of water and antifreeze and are put underground in your garden. The heat from the ground is absorbed into the fluid and is then passed through a heat exchanger into the heat pump. The heat pump can be used throughout the year, even in winter, because the ground stays at a fairly constant temperature underneath the surface. The size of the ground loop depends on the size of your home and therefore how much heat your house needs. Obviously longer loops need more space to be buried in the garden as they draw more heat from the ground. If this is a problem then a vertical borehole can be drilled instead.