The 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. Under the influence of a yellow sun like Earth's, Kryptonians would gain almost unimaginable superpowers: flight, superhuman strength and speed, invulnerability to almost any physical harm, enhanced senses, heat vision, super breath, and freezing breath.
Super-powered Kryptonians were, however, particularly vulnerable to the radioactive remains of Krypton itself — Kryptonite. Also, under a red sun, such as Krypton's native sun Rao, Kryptonians are deprived of their powers.
The Kryptonian race evolved on Krypton until the point where their very bodies drew strength directly from Krypton's sun Rao, giving them amazing abilities, nearly godlike in power. Warfare between the Kryptonians became intense that reached its peak that entered into a near-endless period of conflict known as the Age of Wars that was instigated by Jax-Ur. The warring period was ended by a group of twelve telempathic Kryptonians led by Von-El, whom they defeated Jax-Ur and exiled him from Krypton. The twelve Kryptonians, widely known as the Twelve Wise Ones, marked the end of all warfare between the people of Krypton and leading to the creation of Krypton's ruling body, the Council of Twelve, which ensured that all of Krypton had peace and prosperity for all time.
By the time Rao turned into a red supergiant, it caused the Kryptonians into losing their powers and cooling the planet that succumbs to early ice ages. Because of these drastic environmental changes, Krypton's inhabitants developed advanced technology, enough to halt the tide of glacial ice. Due to their technological advances, the Kryptonians began to explore the farthest reaches of space and establishing a World-Gate network, allowing to explore much of the known universe and contacting many races.
Kryptonians were strictly adhere to a noninterference policy with other alien races in avoiding disastrous consequences known as the Law of Riona Prime. This policy was mainly due to a well-meaning attempt by the Kryptonians to bestow a modified EPM on another race whose planet had been decimated by a comet which resulted in that race destroying itself and their entire planet when different factions fought over the EPM. As such, Kryptonian technology are genetically encoded and tampering by other non-Kryptonians instantly disintegrated.
Regardless of their non-interference clause, the Kryptonians established trade with other space-faring alien races and their World-Gate network allowed that to lead to the creation of the Galactic Confederacy. Though Krypton had never joined the Confederacy, it had retained some friendly relations with it, as well as the Green Lantern Corps after asking them to help enforce their ruling to keep the Gates from being used in war or conquest.
Only one time did the Kryptonians break the Law of Riona Prime was during an inter-dimensional invasion by the Gibborim that they used their advanced weapons in nearly wiping out the Gibborim at the cost of a solar system.
The exploration that the Kryptonians had committed led to being intruded into the endless Kree-Skrull War. The Kryptonians tried to mediate peace for the Kree and Skrull Empires; however, the races refused and tried to ally the Kryptonians upon recognizing that Kryptonian technology was infinitely superior to theirs, and intended to use this to completely annihilate the other. The Kryptonians rejected their alliances and enacted a state of isolationism from the conflict, recalling every Kryptonian from the reaches of space back to Krypton and shutting down the World-Gate network. This action angered the Kree and Skrulls, whom they then declared war on the Kryptonians and forced them to join either side. The Kryptonians, on the other hand, already expected their foes would come for them, laid their efforts in constructing a powerful, orbital battlestation known as Sentrius. This battlestation wiped out millions of Skrull and Kree vessels and ensured that the warring races retreat from conquering Krypton.
By the 20th century, the Kryptonians entered into another conflict with the Shi'ar Empire led by Emperor Kral'Nor over the use of the World-Gate network that Kral'Nor intended on imperial expansion, which the Kryptonians refused. Due to the Kryptonians becoming more restricted to their home planet, the Council of Twelve maintained a defense policy as their military forces fall back to Krypton and constructed a Planet-Shield around the planet. Vastly outnumbered, Krypton was besieged by the Shi'ar for six years. The siege ended when General Dru-Zod and Jor-El brought Sentrius into firing range of the Shi'ar Imperial Navy and destroying every Shi'ar vessel above Krypton. The devastation for the Shi'ar led to Kral'Nor being forced to mediate peace with Krypton.
Just shortly after the end of the Krypton-Shi'ar War, the Kryptonians remained unaware that their nemesis, the Dheronians, took advantage of the peace and teleporting a series of probes deep into the planet's core via Boom-Tube technology supplied by the New God Darkseid, generating a force-field that prevented heat from dissipating to the surface and causing the planet to be unstable and on the verge of a catastrophic explosion. Only the scientist Jor-El foreseen the disaster from his geological surveys of the planet's subterranean layers, but his warnings went unheeded by the Council of Twelve.
Krypton exploded and destroying its once great civilization. Only Jor-El and Lara-El's son, Kal-El, survived and escaped to Earth, where he became the planet's premier hero Superman. But over time, other Kryptonian survivors arise such as Supergirl and Krypto begin to adopt Earth as their new home.