Short answers, pulled from the story.
Ultraviolet radiation is electromagnetic radiation with wavelengths of 100 to 400 nanometers, shorter than visible light but longer than X-rays. Its photons carry energies of roughly 3.1 to 12 electron volts, giving them enough energy to trigger chemical reactions and, at the shorter end of the range, to ionize atoms and damage DNA.
UV radiation was discovered in February 1801 by German physicist Johann Wilhelm Ritter, who observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light. He announced the discovery in a brief letter to the Annalen der Physik.
The division of UV into UVA, UVB, and UVC was decided unanimously by a committee of the Second International Congress on Light on the 17th of August 1932, at the Castle of Christiansborg in Copenhagen.
The atmosphere blocks about 77% of the Sun's UV when the Sun is at zenith. At ground level, UV accounts for only about 3% of sunlight, and more than 95% of that remaining UV consists of longer-wavelength UVA. Almost no UVC reaches the surface at all.
UVB damages messenger RNA in skin cells, triggering a ribotoxic stress response through a protein called ZAK-alpha that produces inflammation and acute sunburn, faster than DNA damage signaling. UVB also forms pyrimidine dimers in DNA; most are removed by nucleotide excision repair using about 30 proteins, but unrepaired dimers can cause mutations. Melanoma in 92% of cases lacks a direct UV signature mutation, suggesting indirect DNA damage independent from UVA plays a major role.
Argon-fluoride excimer lasers at 193 nanometers are routinely used to manufacture integrated circuits by photolithography, with research advancing toward 13.5-nanometer extreme ultraviolet lithography. UV is also used to sterilize surfaces and water, to treat skin conditions including psoriasis and vitiligo, to cure polymer adhesives and inks, and in forensic analysis to detect bodily fluids and authenticate documents.