The series opens with Yuske Urameshi, a 14 year old loner who has no friends, never attends school, and gets into fights just to blow off steam. In a freak accident, he gets run over by a car and dies. He is confronted by the "grim reaper"(really a young lady in a kimono who rides on an oar) who says his death was not supposed to happen, so he gets another chance at lif, but becomes a detective for the underworld. His main weapon is a weapon of concentrated aura blown from his finger as a gun.
He is soon joined by Kuwabara, a fellow classmate who's only goal is to beat Yusuke in a street fight, but never does, Kurama, a collected individual who is "bishonen", and is constantly pushing himself to his own suicide. Basically, Kurama is a masochist. And Hiei, a demon convicted who weilds a sword and constantly wants to taste the blood of his opposers. He never truly sheds his "evil" skin.
Basiclly, after Trigun and Fullmetal Alchemist, Yu yu Hakusho is the best anime ever made.