The Basic Principle
Heating elements are made out of a resistor such as nichrome wire. A resistor is a material which lets electric current flow, but resists the flow.
A large electric current contains a lot of energy. When that current is sent through a resistor, almost all of that energy is turned into heat.
That heat is what is used in electric ovens, furnaces, heat lamps and other heated devices.
The simplest kinds of heaters are convection heaters. In a convection heater, the heat flows into the air directly.
Some convection heaters such as radiators use fins to help distribute the heat into the air. Others, called forced air convection heaters, use fans to blow heat over the heating coil, creating a stream of air that flows through the room.
These heaters depend on air currents or physical contact with an object to move the heat.
All objects release infrared radiation. The hotter the object is, the more infrared is released.
Infrared radiation flows straight through the air, but is absorbed by most solid objects, turning into heat. In infrared heaters, the heating element is encased behind a quartz cylinder.
This cylinder stops heat from leaving the element, heating it up to tremendous temperatures. Although the quartz traps heat, it lets infrared light pass right through.
That infrared hits people, chairs, walls and other objects in the room, heating them directly without heating the air. Since it doesn't depend on heat to transfer the air, an infrared heat lamp can keep you warm even in a cold or drafty room.
Infrared is also sometimes used for cooking, and is important in many industrial processes.