Hell is an interesting subject in Buddhism, though not a particularly important one. It differs from the more Western notion of hell in a number of key ways.
First, the Buddhist “model of existence” is based on ancient Indian cosmology which viewed the universe not a single world, but many different worlds that intertwine, and with living beings trans-migrating between worlds: first one world, then another, all based on one’s karma without end. Buddhism differed from other Indian religions at the time due to its teaching of anātman (roughly “no self”, or “no soul”), but otherwise the basic model was still the same:
These realms collectively were things such as the realm of animals (i.e. pure survival and instinct), the deva realms (realms of heavenly bliss), the human realm, the hell realms and so on. Imagine Dante’s Inferno, in all its agony and dismal torment, but many more layers, and each one is a disparate “world” or “realm” rather than just one layer among several. That is the hell realms (plural) in Buddhism. Some more severe than others, various ironic punishments, etc.
The hell realms, as with the other realms, are all temporary states. One may suffer horrific torment for a long time until one’s karma related to it is exhausted, but eventually it does exhaust and one moves onto another rebirth in some other realm.
However, there’s even more to this story. Another way that Buddhists tend to look at these realms, hell, deva, animal, and so on, is that they are states of mind, just as they are potential destinations for rebirth. The Tiantai (Tendai in Japan) Buddhists first posited an idea that within a single moment of thought, one might dwell in any number of realms. This was called the 3,000 realms in a single thought, to cover all possibilities: past, present or future, and within them, the hell realms, deva realms, basic instinctive survival (animals), and so on. The mind jumps from one state to another, from time period to another, without end.
A person lives in terrible, abusive environment may be said to be living in the hell realms here and now, even if there are breaks of relief in between. A person suffering from a terrible addiction may be living like a hungry ghost.
This notion of the mental realms vs. actual realms of rebirth aren’t necessarily mutually exclusive either. What matters is that there are people around us living in various states, some blissful (even if only temporarily) and others are suffering terrible torment, and everything in between. The Buddhist learns to be aware of this migration from one state to another within their own mind, but also extend this awareness outward towards others around them who may need help or relief.
Namu Amida Butsu
Gleanings in Buddha-Fields 