The Hell Realms are regions of great suffering. Birth here is the result of having performed evil deeds, such as acts of violence or cruelty. The tortures of fire, boiling oil, hooks and swords that beings endure are the direct result of their deeds; this is karma ripening. It should be noted that there is no eternal damnation in Buddhism. Just as in the heavens and on earth, existence here is impermanent. Beings in hell are eventually reborn elsewhere, with the expiration of their old karma.
There are various degrees and levels of hell. The deepest and worst hell is called Avici and here there are only painful sensations without a moment's respite. Since the arising of thoughts of hope would constitute at least a moment's relief, beings here believe their torment to be everlasting.
Hell is ruled by a deity called Yama who judges beings dragged before him by the warders of Hell. It is perhaps significant that he is portrayed as reluctant to admit beings into Hell. He questions each being brought before his judgement seat, and the phrasing of the text seems to imply an exasperation with the perversity of humanity, rather than a wrathfulness.
Then King Yama says "Good man, did it never occur to you - an intelligent and mature man - "I too am subject to death, I am not exempt from death: surely I had better do good by body, speech and mind? He says: "I was unable, venerable sir, I was negligent." Then King Yama says: "Good man, through negligence you have failed to do good by body, speech and mind. Certainly they will deal with you according to your negligence. But this evil action of yours was not done by your mother or your father, or by your brother or your sister, or by your friends and companions, or by your kinsmen and relatives, or by recluses and brahmins, or by gods: this evil action was done by you yourself, and you yourself will experience its result." (Majjhima 130)
One special hell realm is called the Inter-World Hell and is located in the dark and empty void between world systems. Here the chief suffering is loneliness, because of the vast spaces beings may live long ages without encountering another and even come to believe they are alone in the universe.
See Majjhima Nikaya 129 and 130 for the fullest canonical descriptions of Hell.