Yu Yu Hakusho is set in a world where demons are a real threat and humans are kept completely in the dark about it. The world is defended by people in the employ of Koenma, the son of King Emna, ruler of the Spirit World. Yusuke Urameshi is one of those people, acting as a Spirit Detective whose primary focus is keeping the Human and Spirit World safe from spiritual threats. The world is full of psychics, people who have made deals with demons, and even the debauched human elite who make demons the victims for money, power, and pleasure. The world is dark, but there is always hope.
The Spirit World is somewhat of a contrast to the Human World. It is the intersection of the planes, where nearly everything is run efficiently and by the book. Koenma runs this world in his father King Enma's usual absence, deciding where both Demons and Humans go when they die. The Demon World is still considered one of the worlds of the living, it's simply kept separate from Human World for the safety of mankind. But there are always slips, and this is where a Spirit Detective comes in.