Kryptonite is a radioactive mineral from the planet Krypton, which has the ability to weaken Superman. Several variants of Kryptonite exist, some effective, some not so much, and some are just plain, hands-down retarded.
Krypton, the planet where Superman was born, was once a very glorious planet, which civilization was thousands of years more advanced than that of Earth's. Kryptonians had technological marvels such as microbe rays that eliminated all diseases, telepathy helmets that could transfer volumes of knowledge in a matter of minutes, and cloning banks that provided spare body parts, among others.
Compared to Krypton, modern-day humans were armless, legless single-digit IQ'd cavemen.
How Jor-El sees humans, circa 2010.
Krypton, however, was about to explode like a brain that had just finished processing a Stephanie Meyer novel. Jor-El, Superman's father, decided to place him in a rocket, allowing him to escape from Krypton -which by then a war-torn, unruly wasteland filled with Kryptonian civil unrest - and escape to Earth, which, by then, was also a war-torn, unruly wasteland filled with human civil unrest.
Above: an exercise in futility.
Superman, however, discovered that on Earth, he possessed powers any mortal man would envy, such as the capacity to have killer abs, (super-strength), exemption from boo-boos (invulnerability), and most of all, the ability to see through women's clothes (x-ray vision).
Admit it. You'd do this too if you were Superman.
However, the explosion from Krypton's core brought radioactive meteorites to Earth that, if exposed to him for a period of time, would cause him to weaken and die. Since then, Superman has endured a legacy of fighting crime, saving the world, and being paranoid of small, green glowing objects.
Pencillers weren't helping by making Superman look like he was scared to death of cabbage:
"Great Rao! Green...leafy...vegetables!"
Green Kryptonite, the most common variant of the mineral, has the ability to damage Superman, causing poisoning of the blood, inability to use his powers, and general weakness.
Green Kryptonite has been fashioned into many different weapons, such as a cyborg pacemaker-slash-energy blaster, a kryptonite ring, a kryptonite man, a kryptonite man-turned-into-a-laser-beam: You name it, various Superman enemies and plot points have done it.
Except, of course, for actually killing Superman.
Lex Luthor: Wasting Kryptonite on Elaborate Bullshit since the 1950's.
Throughout Superman's red-briefs-wearing career, Lex Luthor and his other enemies such as Metallo, Brainiac, and other members of Superman's rogues gallery have had plenty of chances to murder him.
Inexplicably, they are trade it away for the chance to gloat in front of him, as if the true measure for Metropolis villains is how long they can pad their victory speech before Superman finds a way to escape and haul their respective asses faster than the proverbial speeding bullet.
Above: Metallo, counting off the seconds until he beats Toyman's 3 minute, 42 second monologue.
Action Comics readers' suspension of disbelief are tested to the limit by the Red Kryptonite Scarecrow in Superman costume.
Red Kryptonite is like regular Kryptonite, if its basic premise had been written by a guy two hours into a marijuana session who used a roulette of ideas made by a guy four hours into a marijuana session.
Man...what if, like, we turned Superman into a mood-ring? Heh,heh,heh, yeah...but like, just his face?" (Grabs bag of Cheetos)
The effect of Red Kryptonite in Superman changes so many times during the course of his career, and more often than not transforms his physiology into bizarre and unusual ways. Below are some of the transformations he has experienced due to Red Kryptonite:
- Fire-Breathing Superman
- Fairy Wish-Granting Superman
- Baby Superman
- Face-Mood-Ring Superman
- Giant, Powerless Superman
- Right-Side-Only Invulnerable Superman
- Long-Nailed-Hippie Superman
In this issue, Red Kryptonite puts Superman in peril by transforming him into a half-invulnerable cotton candy man. Comics were simpler during those times.
Each transformation, however, only works once on Superman, after which he becomes immune to that particular piece of kryptonite. The bad thing is, crooks can never know what the effects are either.
This means that if they make the mistake of getting a particular piece Superman has become immune
to, or transfigure him in a way that enables him to use his abilities (note that most of the transformations above are still useful in a fight), all they'll be left with is a red paperweight and the growing realization that they will be facing a twenty-year stint in prison.
Blue Kryptonite is to Superman what a Fire Flower is to Mario, or what a shiv knife is to an ex-convict in prison - that is, it increases their killing capability. Superman, however, never kills.
He's willing to settle for punching Batmen dressed as Santa though.
Essentially the opposite of Green Kryptonite, it strengthens Superman's abilities upon exposure. However, Superman never bothers to keep a hoard of blue kryptonite around, costing him not only a Popeye/Spinach style advantage, but leaving the residents of Bizarro World in danger of the stuff, which is poison to them.
So, essentially, it's "No" to performance-enhancing drugs and "Yes" to genocide. Fuck yes, Man of Steel.
One of the main plot points of CW's Smallville is that green Kryptonite, under certain conditions, gives humans unnatural abilities.
The word 'certain' here is stretched substantially, to the point that the viewer is inclined to believe that even sipping a drink from the local cafe or, say, using a public urinal will turn you into an deranged, meteor-rock infected psychopath.
There's Kryptonite + Pond Water = Quasi-Fountain of Youth...
Kryptonite + Steam Bath = Fire Manipulation...
Kryptonite + Meteor Shower + Young Lex Luthor = Bald Lex Luthor...
...and ten seasons more of similar equations. Considering the bajillions of kryptonite-powered enemy shit he puts up with almost every episode it's no wonder this unverse's Clark Kent often suffers from periods of teenage angst.
A frustrated Clark Kent, after facing a <insert random power here> villain powered by Kryptonite.
The series also features other variants of kryptonite (red, blue, silver), each of them united by their ability to produce crying, tears, break-ups, and other emotional plot lines.
In this case, kryptonite torture pain counts as an emotion.