The Sacred World of the Kais is where the Supreme Kai and his bodyguard Kibito live and eventually Elder Kai and the permanently fused form of the Supreme Kai and Kibito, Kibito Kai. It is possible that the two that Goku freed from the Sugaroku Dimension, Sugoro and Shusugoro, live on the planet of the Kai's, or they may have left to find their old home after the Baby Saga.
The Sacred World of the Kais 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 four moons, which could be a reference to the four Supreme Kais and the whole planet a reference to the Grand Supreme Kai, or it may just be a fluke. Most Kais don't live here, as the Western Supreme Kai and the Northern Supreme Kai were both killed by Kid Buu, whilst the Southern Supreme Kai and the Grand Supreme Kai were absorbed by Kid Buu, during Bibidi's quest to rule the galaxy.
The Planet of the Kai's 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 Goku a while to find Gohan in Other World when he was looking to see if he was dead or not. The planet flourishes with life, with plenty of fish in its streams and rivers, which was shown in the Eastern Supreme Kai's flashback. It also has rocky and grassy areas, as well as icecaps, which was shown in the flashback.
Prior to the series
Approximately five million years before the events of Dragon Ball, the Sacred World of the Kais 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. In a quest to rule the universe, Bibidi unleashed Kid Buu on the peaceful Supreme Kais. Western Supreme Kai and Northern Supreme Kai were the first and second Supreme Kais to be killed by Kid Buu, respectively. Southern Supreme Kai attempted to defeat the Majin, but was absorbed, transforming Kid Buu into his buff Ultra Buu form.
Ultra Buu attempted to kill Supreme Kai, but was stopped by Grand Supreme Kai, who was absorbed by Ultra Buu 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 SagaDuring 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 he claimed could bring out Gohan's sleeping powers.
Main article: Fusion SagaAfter 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 SagaFollowing 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 at the hands of Goku's Spirit Bomb.