The spectrum of electromagnetic radiation is comprised of waves or particles that carry different amounts of energy. The visible spectrum is what humans and most animals see. The least energetic range of electromagnetic radiation is radio. Next is microwave radiation followed by infrared, visible, ultraviolet, x-ray and gamma-ray.
The different types of electromagnetic radiation are determined by the energy content of the particles in that range. This is further governed by the temperature of the medium, according to the Planck blackbody radiation spectrum. The higher the temperature, the more energy the particles or waves have.
Physicists discovered that electromagnetic radiation has a dual-nature as both particles and waves. When viewed as particles, the more energetic they are, the higher in the spectrum they are, up to gamma-rays. When viewed as waves, their length corresponds to their energy content. Radio waves are emitted by radios, low-energy stars and various gases.
Microwaves are also emitted by galaxies as well as the common household heating appliance. Infrared is the radiation of warm bodies and astrophysical dust, heated by higher background radiation. Visible radiation is what human eyes can resolve. Ultraviolet is beyond the blue end of the spectrum and burns humans easily. X-rays are very energetic particles and short-wavelength, which allows them to pass through objects. Gamma-rays have the shortest wavelengths and are emitted by nuclear processes