From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Planet Kaioshin (界王神星, Kaioshin-sei; FUNimation "Sacred World of the Kais"; Literally meaning "World King God Planet") is a planet in the Kaioshin Realm.
Planet Kaioshin is where East Kaioshin and his bodyguard, Kibito, live, and eventually Old Kaioshin and the permanently fused form of East Kaioshin and Kibito, Kibito Kai. It is possible that the two that Goku free from Sugoroku Space, Sugoro and Shusugoro, live on the planet or may have left to find their old homeworld after the Baby Saga.
Planet Kaioshin is where the four Supreme Kais and the Grand Supreme Kai used to live, and possibly every single Supreme Kai has lived. The planet has several moons. Most Kaioshins do not live here as the Western Supreme Kai and Northern Supreme Kai are both killed by Kid Boo, whilst the Southern Supreme Kai and Grand Supreme Kai are absorbed by Kid Boo during Bibidi's quest to rule the galaxy.
Planet Kaioshin seems to be a very empty place. For a large planet, not many reside on it. This may be because of the planet's sacredness or because it is very difficult to get there, as it took Son Goku awhile to find Son Gohan in the Other World when he is looking to see if he is dead or not. The planet flourishes with life, with plenty of Fish in its streams and rivers as shown in the Eastern Supreme Kai's flashback. It also has rocky and grassy areas, as well as icecaps, which is shown in the flashback.
The planet is incredibly sacred, literally to the point where, except in dire situations such as the release of Majin Boo, even the Grand Kais of each galaxy are not allowed to set foot there. This would explain the low population since, prior to the Buu Saga, a maximum of ten people (four Supreme Kais, one Grand Supreme Kai, and the bodyguards of each) were allowed to be there at all.
Prior to the series
Approximately five million years before the events of Dragon Ball, the Kaioshin planet was inhabited by all the Supreme Kais of the universe: Grand Supreme Kai, Northern Supreme Kai, (Eastern) Supreme Kai, Western Supreme Kai and Southern Supreme Kai. According to the Eastern Supreme Kai (who, upon the deaths of every other Supreme Kai, became the Grand Supreme Kai), all five of them were one thousand times stronger than Freeza. In a quest to rule the universe, Bibidi unleashed Kid Boo on the peaceful Supreme Kais. Western Supreme Kai and Northern Supreme Kai were the first and second Supreme Kais to be killed by Kid Boo, respectively. Southern Supreme Kai attempted to defeat the Majin, but was absorbed, transforming Kid Boo into his buff Ultra Boo form.
Ultra Boo attempted to kill Supreme Kai, but was stopped by Grand Supreme Kai, who was absorbed by Ultra Boo moments later. Afterward, Ultra Buu transformed into the childish and fat Majin Buu, disrupting his reputation as a cold and efficient killing machine.
Majin Buu Saga
Main article: Majin Buu Saga
During the Majin Buu Saga, Kibito used Instantaneous Movement to transport himself, Supreme Kai and Gohan to the Sacred World of the Kais so that Gohan could extract the Z Sword from its resting place and train with it. Upon returning to Other World after his one-day pass to Earth had expired, Goku was able to sense Gohan and travel to the Sacred World of the Kais using Instant Transmission. During the course of Gohan's training, Goku and Gohan inadvertently released Elder Kai from his imprisonment in the Z Sword. Elder Kai then went on to begin a long and drawn-out ritual that would unlock potential within Gohan.
Main article: Fusion Saga
After many hours, Elder Kai succeeded in awakening Gohan's dormant powers. Kibito then returned Gohan to Earth so that he could confront Super Buu. When it became apparent that Gohan would be unable to defeat Super Buu (who had then absorbed Piccolo and Gotenks), Elder Kai taught Supreme Kai, Kibito and Goku how to fuse using Potara Earrings. Supreme Kai and Kibito eagerly and successfully tested this ability, fusing to become Kibito Kai. He was later appalled to discover that the Potara fusion was permanent, unlike the fusion induced by the Fusion Dance. Despite its apparent irreversibility, Goku felt that his fusing with Gohan was their only way to defeat Super Buu and returned to Earth with a pair of the Potara Earrings.
Kibito Kai and Elder Kai both continued to observe the unfolding of events from the Sacred World of the Kais, using a crystal ball.
Kid Buu Saga
Main article: Kid Buu Saga
Following Kid Buu's destruction of the Earth, Goku returned to the Sacred World of the Kais, accompanied by the only surviving refugees from Earth, Dende, Vegeta, Mr. Satan and Bee. In an attempt to lure Kid Buu away from his wanton destruction of other planets and their inhabitants, Goku and Vegeta summoned up energy and raised their power levels. Kid Buu detected them, and used Instant Transmission to travel to the Sacred World of the Kais. Anticipating the imminent battle, Kibito Kai, Dende and Elder Kai escaped to a faraway planet on the physical plane using Kibito Kai's Instantaneous Movement, but unintentionally neglected to bring Mr. Satan and Bee.
Goku and Vegeta proceeded to fight Kid Buu on the Sacred World of the Kais, ultimately culminating in Kid Buu's defeat by Goku and his Super Spirit Bomb however the battle largely damaged the planet much to Kibito Kai's distress.
Dragon Ball GT
The Sacred World of the Kais is used as a safe place for Goku, to hide away from Baby. Goku's tail is regrown, afterwards going to the newly restored Planet Tuffle where Goku turns into a Golden Great Ape (and eventually a Super Saiyan 4). The planet doesn't hold much significance in the Dragon Ball plot line after that, though it continues to serve as a residence for Kibito Kai. It is also where Kibito Kai and Old Kai witness the final battle between the Super Saiyan 4's, Goku and Vegeta, against Omega Shenron.
The Sacred World of the Kais has appeared in numerous video games as stages, however, the planet is called "Supreme Kai's planet" instead of the Sacred World of the Kais. It has appeared in Budokai 2, Budokai 3, Infinite World, Budokai Tenkaichi, Shin Budokai, Budokai Tenkaichi 2, Budokai Tenkaichi 3, Raging Blast and Raging Blast 2.