Selenium resists electricity depending on the amount of light hitting it, making it ideal for use in a variety of gadgets, although not as useful as the substances that replaced it. Selenium has similarities to arsenic, first and foremost, it is deadly as well as stinky. However, like the gadget thing, it is not as poisonous as most of the things it takes the place of. In small amounts, moreover, Selenium is vital to human life. Selenium is God's elemental half-measure.
Selenium was discovered in 1817 by Jöns Jacob "Wrong Way" Berzelius. He was part owner of a sulfuric acid plant in Sweden with a process that created a red precipitate he assumed was an arsenic compound. That is probably why he took a good long whiff of it and reported that it smelled like horseradish. Arsenic didn't smell like that. However, tellurium, which had just been invented, did have a stench to it. Luckily for posterity, he did not attempt a taste-test instead.
He started writing to friends about his tellurium find. They wrote back insisting that there was no damned tellurium in the ore mines. Berzelius analyzed the red precipitate further and decided he had a newly discovered element of his own. Naming the stuff Berzeliusium turned out to be a non-starter in the chemistry community. Therefore, as tellurium was named for the Earth, he named his new stinky stuff after the moon (σελήνη, selene), as everything on the moon is assumed to smell worse than it actually smells if you take off your space helmet and breathe it in.
Where to find it
Back on Earth, selenium is found mostly in metal sulfide ores, on the occasions that sulfur is taking the day off. Commercially, plants refining these ores find they have a lot of selenium on their hands that they have to get rid of.
Likely chumps include those in the glass-making and paint business. Paint makers like it because it is, after all, red. Glass makers like it for the same reason; conferring a red tint to glass is actually a good thing when the glass contains iron, which confers a green or yellow tint. Once again, selenium comes through as less annoying than the thing it's replacing. For this reason, the glass business accounts for half of all selenium use.
In the human body
On the contrary, you do not find selenium in the supermarket, because even small doses of selenium salts are deadly, leading to selenosis, which means inflammation of the selenium. The U.S. Food and Drug Administration has wiped selenium from all four Basic Food Groups and has set an upper limit of 400 micrograms per day. This level was based on a 1992 study of five Chinamen who ate corn grown in selenium-rich soil. Their various maladies could not have resulted from the corn, or else the entire population of Tennessee would have been poisoned by moonshine, so it must have been the selenium. (The study actually determined that 800 micrograms was a safe level, but the FDA pulled a lower number out of its arse. Not to be outdone, the Occupational Safety and Health Administration has chosen a safe level of 200 micrograms, using the same technique.)
In the body, selenium is found in several amino acids, where it plays a role analogous to that of sulfur, viz., smelling bad. Despite the ill effects of overdose, selenium is an essential nutrient. That is, you can't live with it and you can't live without it.