Radiation is the transfer of heat by electromagnetic radiation. Just like visible light, heat radiation can travel with or without a medium. This is how heat from the sun travels through space, which is a vacuum (or the absence of matter). Heat radiation is in the infrared range of electromagnetic radiation with a frequency slightly lower than visible light, but higher than radio waves. The more energy the hot object has the more heat it radiates, and the higher the energy of that heat. As an object gets hotter, some of the radiation extends out of the infrared and into the visible.

Source: Hot Metalwork, Fir0002, Wikimedia Commons

Most light bulbs give off some infrared radiation, or heat. The following video shows a radiometer reacting to a bright light source. The black side of each arm of the radiometer absorbs heat and causes the gas next to it to expand. This expansion causes high pressure which makes the radiometer spin.