What can make thermal radiation confusing is its relationship to heat. Heat can be transferred in three ways: convection, conduction, and radiation. Conduction deals with objects touching; convection deals with liquids and gases moving to spread temperature. Conduction and convection aren’t important for the purposes of this discussion.
Imagine a cup of hot coffee sitting on a table. The coffee is hotter than the air around it, but eventually the coffee will cool off. This is because the coffee is transferring heat to the air around it through the process of thermal radiation.
Because the coffee is hot, the particles inside are moving around fast, meaning they are very excited. Because they’re excited, they keep releasing photons. Some of these photons go through the coffee cup and out into the air, hitting particles in the air. The air particles get energy from these photons hitting them. That energy causes the air particles to get excited, causing them to move faster—thus heating up the air. The coffee is continually losing photons to the air—the air is also losing photons to the coffee, but the air has less energy because it is at a lower temperature. Thus, the air warms while the coffee cools. Eventually the air and the coffee reach equilibrium, meaning the air and the coffee are the same temperature.
The objects around you are all emitting electromagnetic radiation as a result of thermal energy; that radiation is exciting particles in other objects, thus heating them. Therefore, people will often say that the electromagnetic radiation emitted as a result of thermal energy—usually infrared—is heat. In fact, people conflate all of these terms: electromagnetic radiation, infrared radiation, thermal radiation, thermal energy, temperature and heat. Understand that the following terms all refer to slightly different things:
Electromagnetic radiation – radiation on any part of the electromagnetic spectrum
Infrared radiation – electromagnetic radiation on the infrared part of the spectrum, often but not always produced by thermal energy
Thermal radiation – electromagnetic radiation produced by thermal energy, often but not always in the infrared part of the spectrum
Thermal energy – the energy an object has as a result of kinetic energy. Always produces thermal radiation, often but not always in the infrared part of the spectrum.
Temperature – the average kinetic energy of an object. Objects with a temperature always have thermal energy and so always produce thermal radiation. The hotter an object is, the higher the frequency of the thermal radiation produced.
Heat – the transfer of thermal energy between objects of different temperatures, which can occur via thermal radiation, but can also occur in other ways.
The electromagnetic radiation that objects produce as a result of thermal energy is sometimes called blackbody radiation. Technically, blackbody radiation is the radiation emitted by an object whose only source of electromagnetic radiation is its thermal energy (i.e., the movement of its particles, which gives us not only thermal energy but also its temperature). In reality, most other objects have other properties unrelated to their thermal energy that also cause them to emit electromagnetic radiation. These other properties are how reflective, absorptive, diffractive, scattering, or refractive the object is.