Technology behind the system is very simple.  Low grade heat is transferred from the ground to a heat pump where it is then increased in temperature before it is transferred to the heating distribution system. 
Your domestic fridge uses the same technology. As you put food and drink in to your fridge the low grade heat it carries (after all it's usually warmer than the inside of the fridge) is transferred from the ice box to the refrigerant in the unit. The refrigerant is then compressed and expanded to raise the heat, this high grade heat is then expelled from the back of the fridge. This is why the inside of the fridge remains cold whilst the back of the fridge gets hot.