A central heating system that transfers heat from a boiler to each room through a fluid filled plumbing system.
A central heating system that transfers the heat from the boiler via hot water to each room. The air in the rooms is then heated using radiators (normally passive) that have the hot water flowing through them. Due to the low temperature of Radiators, most heat is given out by convention not radiation.
Sometimes the floor will be used as large radiators; this is called under floor heating (UFH)
These days it is normal for the hot water to be pumped through the system.
Water based central heating systems are the norm in the UK, however in the USA it is normal for the boiler to directly heat air that is then ducted into each room.