Hell is the destination of those who committed unforgivably evil acts during their lives in the human world. When a Hollow whose mortal soul is too wicked to enter Soul Society is slain by a Zanpakutô, the gates of hell (giant doors held by skeletons) appear and begin to open. A giant, laughing spiritual being with a blade spears the wicked spirit and drags it down into hell.
There exists a King of Hell, who's power and motivations are unknown. No one who has ventured into Hell for the sake of research has ever returned, so the exact contents and nature of the realm is also unknown.