Short answers, pulled from the story.
The electromagnetic spectrum is the full range of electromagnetic radiation organized by frequency or wavelength. From low to high frequency it runs through radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays, all traveling at the speed of light.
Ordered from low to high frequency, the bands are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. The bands have no sharp boundaries and fade into one another like the colours of a rainbow.
William Herschel discovered infrared radiation in 1800 by measuring temperatures of light split through a prism and finding the highest temperature beyond red. The next year Johann Ritter found "chemical rays" beyond violet that were later renamed ultraviolet radiation.
During the 1860s, James Clerk Maxwell developed four equations for the electromagnetic field, two of which predicted waves. When he found those waves must travel at about the speed of light, he inferred that light is itself a type of electromagnetic wave, the first indication of an entire spectrum.
Ultraviolet, X-rays, and gamma rays are collectively called ionizing radiation because their high photon energy can ionize atoms and cause chemical reactions. Longer-wavelength radiation such as visible light is non-ionizing, since its photons lack sufficient energy to ionize atoms.
X-rays and gamma rays are distinguished by source rather than energy. Gamma rays come from nuclear decay and other nuclear processes, while X-rays come from electronic transitions involving deep inner atomic electrons. In astrophysics, energies below 100 keV are called X-rays and higher energies are gamma rays.