The warmth is delivered by moving the heated water through a network of pipes around the house, usually divided into separate zones on each floor. Radiating outwards from the water into the air in each room, the heat is accelerated and amplified by fins in the baseboard or reflectors in the floor. It is a passive, unobtrusive friend.
In older houses, this system is identified by the ornate bulky cast-iron boilers in every room. Homes less than 50 years old have sections of bulky baseboards along the wall, noticeable only as limiting for the placement of furniture.
In the basement, the efficient system sits contained in a small box (size of a filing cabinet) with a well-organized mane of pipes and valves. Often now, the hot water tank for dishes and showers serves as auxiliary to hold water heated by the boiler, no longer a heater by itself.
This system is the most expensive to install and cheapest to maintain, quietly (well, older versions provide the comforting gurgle and knocks moving through) providing an even, warm heat without another thought. Anyone who just wants to go about their day should live with hot water heat.
Baseboards, like grilles of a forced air system, are usually set under windows to offer the strongest defense against the largest loss. A large room requires longer runs, limiting some locations for a sofa or bed. The pipes can remain effective built into a shelf or cabinet unit.
Although the most expensive installation, most people would prefer to run radiant in the floor, especially in a solid slab concrete floor. This requires a web of pipes weaving to cover every square foot of the floor for maximum warmth and effect. The mass becomes warm enough to live in bare feet while winter rages outside, heating from your toes upwards.
Beyond cost, there is no downside to hot water heat. Solid and persistent, this is a heat source that does all the work with little complaint beyond the creaking of expanding pipes. There are no pauses in the day to feel cold or hot; simply set the thermostat and live.