Thermal radiation is a mechanism for the transmission of thermal energy. Unlike convection and conduction heat radiation occurs also in a vacuum. It is merely attached to the possibility of propagation of electromagnetic waves.
Colloquially heat radiation is usually understood to only be the non-visible infrared portion of the thermal radiation.
Radiant heat travels through air almost without loss and heats only when hitting the walls, floor or body. These in return radiate heat themselves. While in convection and conduction (by air), the thermal energy flows in the form of molecular motions from the location of higher temperature to the lower one with radiant heat the energy transport takes place in the form of electromagnetic waves (infrared radiation), that means no heat transfer medium.
The radiated heat energy emitted by a surface is disproportional to the temperature difference. This means that by doubling the temperature of the heat source the emitted radiated heat is increased to about 16 times.
Heat radiation passes through air with almost no loss and tempers walls, machines, storage items, flooring when struck who subsequently release heat back into the environment. Of course also a person who is located within the radiation exchange with the heat source perceives a direct heating. The absorbed radiant heat is felt quite comfortable despite low air temperatures. Examples: Radiant heat of a fire in open air, warmth of the suns radiation on the ground or directly on the skin such as sun in the snow.
The efficient principle of infrared radiation is used commercially in luminous and tube space heaters. Infrared radiant heaters are decentralized heating systems who’s energy is converted into heat without energy loss during transport contrary to central heating (eg by warm water from the boiler to the heating surface)