Kryptonians are an alien humanoid race of being who originate on the planet Krypton.
Kryptonians are basically humans in their structure and appearance. But their biology is far more advanced than that of Earth based humans where they are able to utilize energy in their body with far more direct and elaborate manners than Earth based humans do.
Various dimensional Kryptonians have developed superpowers in regards to Earth based humans that vary drastically in comparison to their various native dimentions. Super speed, heat vision, x-ray vision, and super breath are among many Kryptonian superpowers.
Earth-One dimension Kryptonians' bodies are able to directly absorb and process yellow solar radiation through their body for a host of effects that would allow them true infinite power levels when fully charged. Red sun radiation would not only interfere with that absorption but directly drain Earth-One Kryptonians of all their powers. Superman (Earth-One)
Earth-Two dimension Kryptonians' bodies had advanced to the point were they are able to directly produce these energies for effect on their home planet directly in the red sun system which was vey different from most other known Kryptonians. Earth-Two dimensional Kryptonians naturally produce and maintain these power levels regardless of being in a yellow or red sun system. Originally the level of power that Earth-Two Krytonians had in comparison to other dimensional Kryptonians were far lower than those of other dimensions such as the Earth-One dimension. However some Earth-Two dimension Kryptonians have advanced to higher levels but are usually far less than those of the other dimensions such as Kal-L and Power Girl.
Earth-Three dimension Kryptonians' bodies gained their superhuman powers through the energies of Kryptonite rather than that of a yellow sun. They would lose their powers if seperated from the energies of Kryptonite but the process of leaking was slow and would take time to completely drain off.
Earth-Prime Kryptonians' bodies like those native to the Earth-One dimension are able to directly absorb and process yellow solar radiation through their body for a host of effects that would allow them true infinite power levels when fully charged. But unlike Earth-One dimension Kryptonians on which red sun energy would directly drain Earth-One dimension Kryptonians, Earth-Prime Kryptonians would not be charged any longer. While Earth-Prime Kryptonians would also lose their powers in a red sun system the drainage would not be immediate. Superboy-Prime
Kryptonians from the Time-Trapper Pocket Dimension functioned almost identically to that of the Earth-One dimension and had literally almost infinite power abilities Superboy (Pocket Universe)
The term Kryptonian can also refer to anything created by or associated with Krypton. For example, in the CW TV Series Smallville, Professor Milton Fine (aka Brainiac) is not really a kryptonian but an advanced artificial intelligence created by Kryptonians. Kryptonians are vulnerable to kryptonite.
Culture
Most Kryptonians are usually portrayed as a highly advanced social and generally technologically advanced people. Depending on the dimension considered, Kryptonian technology reflects different base some inorganic as in the Earth-One dimension while the current DC reality seems to be based on organic structures such as various crystaline structures.
In the movie versions of Krypton the "Fortress of Solitude" is a recreation of Krypton’s surface. There are at least 28 known galaxies known to Krypton as revealed by Jor-El in the movie Superman.
Survivors
When Krypton was destroyed, it was thought that the entire kryptonian race was destroyed. This basis is largely disproven with not only the most popular known survivor of Kal-L and Kal-El who was sent off Krypton to Earth by their fathers right before their source worlds were destroyed. Kal-L grew up on Earth as Clark Kent and eventually discovered his kryptonian origins, where Kal-El grew up on Earth aware of his Kryptonian heritage. The current incarnation reflects more of Kal-L being solid in the idea that while biologically Kryptonian considers himself as a human culturally.
Kal-El and Kal-L were NOT the only survivors of their respective home worlds. In most dimensions, their cousin Kara ( Power Girl in Earth-Two dimension and Supergirl in Earth-One dimension and current reality) would also be sent from Krypton to Earth. In most dimensions, the entire city of Argo would also survive the destruction of Krypton either being thrown off the exploding planet or as a space colony away from the source planet.
In the film Superman, young Clark discovers who he is in the Fortress of Solitude, where a crystal hologram of Jor-El tells him "You are the only survivor of the planet Krypton." The last remark also appeared to prove untrue as prisoners of the Phantom Zone such as General Zod also survived the destruction of Krypton. Supergirl, cousin of Superman, also survived Krypton's destruction and made home on Earth. The capital of Krypton, Kandor, was also spared from destruction as it was shrunken by Brainiac.
See Krypton