Heating radiators are powered either by steam or electricity. In a steam powered radiator, a boiler heats water until it turns into steam. The steam flows up through a system of pipes which channel it into radiators. When the steam is inside the radiator, some of its heat flows into the metal appliance. The radiator uses this heat to warm the room. Meanwhile, the steam in the radiator cools back down to water and trickles back to the boiler to gather more heat.
Electric Powered Radiators
Electric radiators use heating elements similar to those found in electric stoves, water heaters and other devices which generate heat by electricity. Electric current is a type of energy. When electricity flows into the heating element, the element resists the flow of the current, turning the electricity into heat. That heat then flows outward through the body of the radiator, which it sends back into the room.
Radiators do distribute some radiant infrared heat, but they actually do most of their heating through convection currents. Heat flows from the radiator into the air directly around it, causing that air to expand and rise. The warm air flows into the room, then cools and sinks down. As the warm air moves away from the radiator, it leaves a vacuum behind it which pulls cool air from the ground towards the radiator, where it is warmed. This creates a convection current--a continuous stream of air which flows up and away from the radiator, then cools and flows back towards it where it is heated again. This convection stream distributes heat through the whole room.
Warm objects emit infrared radiation. The warmer the object, the stronger the radiation. The infrared radiation travels through air without heating it, but when it strikes something solid, it warms that object. This is called radiant heating. If you stand next to a radiator, you can feel the infrared radiating off of it. Although most of the actual heat flows upwards with air currents, the infrared radiation flows in all directions, which is why you can feel it from the side. This helps to heat the room and the people and objects in it.