Demon World (魔界, Makai) is the world where demons live. It is an alternate dimension that King Enma sealed off with a powerful barrier which forbade any demon ranked A Class or higher to cross. For many years it was ruled by 3 kings: Toshin Raizen, Mukuro and Yomi, though not in the literal sense. There was no set government, only territories over which the 3 demons ruled; these territories were formed out of the natural chaotic state of the environment. Despite the order that they brought in accordance to the times and intricacies, they could be attacked at any time and no rules would be broken because there weren't any to begin with. In other words: the rule over a territory all came down to who had the greatest strength and ambition, much akin to the real-life criminal underworld.
Each territory had its own individual laws, as established by its rulers, until the establishment of the Demon World Tournament, whose victor would unify Demon World under him/her as the supreme ruler. Raizen, Mukuro, and Yomi are acknowledged as the three kings because out of all the kingdoms in the land, they have the greatest influence and strength to negotiate a deal with King Enma to allow Yusuke, Kurama, and Hiei to return to Demon World.
Demon City, which is occupied by the Four Saint Beasts, and the Island of the Koorime, where Yukina is from, are distinct territories that did not appear to be under direct control of any of the 3 demon kings.
The Forest of Fools, an area mentioned by Hiei, is a territory where civilian Youkai live peacefully.
According to Koenma, Demon World is rather like an endless basement, with its strongest demons living deeper and deeper within its depths. With its surface inhabited by demons of fair power and notoriety, demons within its ultimate depths are said to be god like and mere embodiments of destruction.
The sky in Demon World seems to be crimson, with dark clouds during daytime. Much of this world seems to be terrains, including untouched wastelands, forests, mountainous regions, deserts etc. However, despite this natural and primitive environment, Demon World's technology surpasses that of Human World, as healing chambers and artificial birth machines have been employed by the three kings on different occasions. Also, Demon World seems to have cities with technologies such as television as seen during the Demon World Tournament.