This house is heated by forced hot water. Natural gas is burned to heat the water stored in a loop that goes to the radiators in the house. When the thermostat recognizes that the room temperature has fallen below a set limit, a signal goes to the furnace, which turns on and heats the water. Heated water then flows through the pipes to the radiators. The radiators are heat exchangers, and transfer the heat into the air in the room. When the temperature at the thermostat reaches the preset level, the system stops sending hot water to the radiators.
One of the rooms in this house is equipped with radiant heat in the floor. Below the bricks, is a network of plastic tubing, which carries the hot water from the furnace. The heat is transferred out of the tubing, into the brick of the flooring, and then into the air of the room.
There are pictures of this system.