Short answers, pulled from the story.
Electromagnetic radiation is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It spans a broad spectrum classified by frequency, from radio waves, microwaves, infrared, and visible light up to ultraviolet, X-rays, and gamma rays. All forms travel at the speed of light in a vacuum and show wave-particle duality.
Electromagnetic radiation travels through a vacuum at the speed of light, 299,792,458 meters per second, about 186,000 miles per second. In any medium other than vacuum it moves slower, set by the medium's refractive index, because its fields polarize the charged particles they pass through.
Electromagnetic radiation shows wave-particle duality, meaning it exhibits wave and particle traits at the same time, both confirmed in many experiments. Wave traits dominate over large distances and long timescales, while particle traits appear over small distances and short timescales. The double-slit experiment has demonstrated this duality for a single real photon.
A photon is an uncharged elementary particle with zero rest mass that is the quantum of the electromagnetic field. Max Planck proposed energy quanta in 1900 and Albert Einstein argued in 1905 that these light quanta are real particles. A photon's energy is proportional to its frequency through the Planck constant.
Invisible electromagnetic radiation was discovered in the early 19th century, beginning with William Herschel detecting infrared beyond the red of the spectrum in 1800. Johann Wilhelm Ritter found ultraviolet in 1801, James Clerk Maxwell developed his field equations in 1862-64, and Heinrich Hertz deliberately produced radio waves in 1887. Wilhelm Rontgen discovered X-rays in 1895.
The danger depends on the radiation's power and frequency. The World Health Organization classes all ultraviolet frequencies as Group 1 carcinogens, and ultraviolet from the sun is the primary cause of skin cancer in fair-skinned people. X-rays and gamma rays are ionizing and can cause mutation and cancer deep below the skin, while radiofrequency radiation is classed as Group 2B, possibly carcinogenic.