The term hydronic heating systems covers several different systems, each with their particular advantages and shortcomings.
All of them are forced hot water heating systems - that is to say they work by heating water to high temperatures and then the push it around the house where it gives up any heat it has to the rooms it passes through. The forced bit is because it is necessary to pump the hot water around the house.
In fact most types of hot water heating only have three important components - the heating part (the boiler), the radiators or heat pipes, and the pump.
There are two main types of system, the first uses a visible radiator - usually wall hanging or at floor level, and the second type are under floor hot water heating systems.
The baseboard hot water heating systems or wall hanging radiators are fairly straightforward to fit into any home - it is one of their advantages.
Hydronic radiant floor heat is lovely, but for some home layouts it can be a little more difficult to fit since it needs to go beneath the floor.
Both systems can be powered by any kind of boiler - there are gas, wood, oil, electric and coal fired hot water heating systems.