The Makai (魔界, Demon World) is the world where Youkai (妖怪, "Demons") live. It is an alternate dimension that King Enma sealed off with a powerful barrier which forbade any Youkai ranked A-Class or higher to cross. For many years it was ruled by the Three Demon Kings; Raizen, Yomi, and Mukuro (Apparently, being a ruler in the Makai is not a birth right, but a status that comes with a rise to power - a similar system to the criminal underworld.) Each territory had its own individual laws, as established by its rulers, until the establishment of the Makai Tournament, which created a high Demon King to unify the Makai under one supreme ruler.
Raizen, Mukuro, and Yomi are acknowledged as the three kings because out of all the kingdoms in Makai, they have the greatest influence and power to a degree where they can negotiate a deal with King Enma to allow Yusuke, Kurama, and Hiei to return to demon world, yet another example of the criminal underworld system.
Note: Makai as a whole was not literally controlled by the three kings. Each "King" only controls territory under their rule, and example of this is the city Gandara, the Territory under Yomi's control and Alaric, under Mukuro's control. However, there is nothing stopping any of the kings from expanding their territory, with each having enough power and influence to do so.
Demon City, which is occupied by the Four Saint Beasts , and the Island of the Koorihime, where Yukina is from, are also distinct territories that did not appear to be under direct control of any of the Three Demon Kings.
The Forest of Fools, an area mentioned by Hiei, is a territory where civilian Youkai live peacefully.
The sky in Makai seems to be crimson, with dark clouds during daytime. Much of Makai seems to be terrains such as untouched wastelands, forests, mountainous regions, deserts, and other such enviornments. However, Makai's technology is as high, and surpasses that of Ningenkai, as healing chambers, as well as artificial birth machines have been employed by the three kings on different occasions. Also, Makai seems to have cities with technologies such as television as seen during the Makai Tournament.
Inhabitants Include: Jaganshi Hiei, Yukina, Youko Kurama, Yomi, Mukuro