Kryptonite is a fictional element in the DC Comics universe, and one of the few items that can harm Superman, his cousin Supergirl, and his dog Krypto.
It is depicted as a radioactive fragment from the planet Krypton, able to kill the Man of Steel while staying in contact for a long period of time. Some of its notable users are Lex Luthor and Metallo.
Kryptonite comes in many different forms, though the most infamous is Green Kryptonite. Each Kryptonite variant has vastly different properties but are in general harmful to Kryptonians.
However, Kryptonite is unique to each universe, meaning Kryptonite from Earth-1 will not work on Earth-3, Superman, or vice-versa. This came into especially great effect with Superboy-Prime, whose universe lacked Kryptonite, and thus made him one of the few Kryptonians in existence with a complete immunity to Kryptonite.
Green Kryptonite is the most common and iconic state of Kryptonite. It causes great discomfort and weakness in Kryptonians, as well as radiation poisoning in other life-forms. For example, Lex Luthor contracted cancer from prolonged exposure to Green Kryptonite.
Gold Kryptonite has the ability to instantly depower Kryptonians. Unlike Green Kryptonite, prolonged exposure to Gold Kryptonite will permanently depower Kryptonians, and deny them the ability to gain superhuman powers. In effect, this turns them into ordinary humans.
Red Kryptonite changes Kryptonians personality into evil which makes him lose all inhibitions and becomes unpredictable of his actions.
Black Kryptonite causes Kryptonians to split into two different entities, one embodying the "evil" side of the personality.
The material has been adapted to many forms of media, including toys, movies, television shows, and video games.