The realm of the Supreme Kai in its entirety is 1/10th the size of the universe.
The Sacred World of the Kai is where the four Supreme Kais and the Grand Supreme Kai used to live, and possibly where every single Supreme Kai has lived. Most Supreme Kais do not live here anymore, as the Western Supreme Kai and the Northern Supreme Kai are 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.
During the time the Dragon Ball series is set, the Sacred World of the Kai is home to the Eastern Supreme Kai and his bodyguard Kibito, and later to Old Kai and the permanently fused form of the Supreme Kai and Kibito, Kibito Kai. The two Space Lemurs that Goku freed from the Sugoroku Space, Sugoro and Shusugoro, also live on the planet of the Kais during and after the Baby Saga.
The planet has several moons and 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 was shown in the flashback. The planet of the Kai is incredibly sacred, literally to the point where, except in dire situations, such as the release of Majin Buu, even Grand Kai is not allowed to set foot there. The planet seems to be a very empty place for a large planet, as not many reside on it. This may be because of the planet's sacred value, 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.
Prior to the series
Approximately 75 Million Years prior to the events of Dragon Ball Z this planet was inhabited by Old Kai who lived there peacefully until one day an Old Witch appeared and took of one of his Potara Earrings accidentally, merging the two together permanently. Another day the God of Destruction Beerus dropped by for an administrative meeting being held once every 1,000 years between the Gods of Destruction and the Supreme Kais, and the two of them got into an argument over some trivial matter causing Beerus to get mad and seal him up inside the Z Sword. Approximately five million years before the events of Dragon Ball Z, the Sacred World of the Kai was inhabited by all the Supreme Kai 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 much stronger than Frieza. In a quest to rule the universe, Bibidi unleashed Kid Buu on the peaceful Supreme Kai. 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 Eastern 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.
Dragon Ball Z
During the Majin Buu Saga, Kibito transports himself, Supreme Kai and Gohan to the Sacred World of the Kais thanks to his Instantaneous Movement, so that Gohan can extract the Z Sword from its resting place on the Z Sword Plateau and train with it. Upon returning to Other World after his one-day pass to Earth had expired, Goku is able to sense Gohan's ki and travels to the Sacred World of the Kais using Instant Transmission. During the course of Gohan's training, Goku and Gohan inadvertently release Old Kai from his imprisonment in the Z Sword. Old Kai then goes on to begin a long and drawn-out ritual that would unlock potential within Gohan.
After many hours, Old Kai succeeds in awakening Gohan's dormant powers. Kibito then returns Gohan to Earth so that he could confront Super Buu. When it becomes apparent that Gohan would be unable to defeat Super Buu (who had then absorbed Piccolo and Gotenks), Old Kai teaches Goku how to fuse using Potara Earrings. Supreme Kai and Kibito eagerly and successfully test this ability, fusing to become Kibito Kai. He is later appalled to discover that the Potara fusion is permanent, unlike the fusion induced by the Fusion Dance. Despite its apparent irreversibility, Goku feels that his fusing with Gohan is their only way to defeat Super Buu and returns to Earth with a pair of the Potara Earrings.
Kibito Kai and Old Kai both continue to observe the unfolding of events from the Sacred World of the Kai, using a crystal ball.
Following Kid Buu's destruction of the Earth, Goku returns to the Sacred World of the Kai, accompanied by the few 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 summon up energy and raise their power levels. Kid Buu detects them and teleports to the Sacred World of the Kai. Anticipating the imminent battle, Kibito Kai, Dende and Old Kai escape to a faraway planet on the physical plane using thanks to Kibito Kai's Instantaneous Movement, but unintentionally neglected to bring Mr. Satan and Bee.
Goku and Vegeta proceed to fight Kid Buu on the Sacred World of the Kai, ultimately culminating in Kid Buu's defeat by Goku and his Super Spirit Bomb. The battle largely damaged the planet however, much to Kibito Kai's distress.
Dragon Ball GT
The Sacred World of the Kai 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 does not 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.
Dragon Ball Super
The Sacred World of the Kai is a battle stage in Dragon Ball Z: The Legend, Dragon Ball Z: Legendary Super Warriors, Dragon Ball Z: Supersonic Warriors 2, Dragon Ball Z: Budokai 2, Dragon Ball Z: Budokai 3, Dragon Ball Z: Infinite World, Dragon Ball Z: Shin Budokai, the Budokai Tenkaichi series, the Raging Blast games, Dragon Ball Heroes, Dragon Ball Kai: Ultimate Butōden, Dragon Ball Z: Ultimate Tenkaichi, Dragon Ball Z: Battle of Z, Dragon Ball: Ultimate Swipe, and Dragon Ball: Xenoverse.
It is visited in Dragon Ball Z: Buu's Fury. The store containing some of the best items in the game (such as the Kiloton Gloves/Boots) is on this planet, and can be accessed once Buu is defeated by breaking through the crack in the wall to the right-most map.