A hell dimension is a dimension with conditions extremely hostile for the development of human life and in which demons are the dominant life form.
In relation to Earth
There are thousands, perhaps millions, of other worlds besides the Earthly dimension. Many of these worlds are inhabited by pureblood demons, and are known as "hell" dimensions due to their similarity to the nightmarish afterlife believed to exist by many religions. Years ago, Earth was also home to pureblood demons known as the Old Ones, but they were driven out of this dimension by man. Even since then, the Old Ones have been trying to break the barriers between dimensions and reclaim Earth as their own. Areas in which this barrier has grown thin are known as Hellmouths; mystical convergences which emit supernatual energy and attract evil. The barrier can also be breached by performing certain rituals at "hot spot" locations. An ancient, mystical force known as The Key is capable of breaking down these barriers completely, causing all dimensions to bleed into one another and chaos to reign.
Types of hell dimensions
The hell dimension which Jasmine once ruled over.
There are many types of hell dimension. Some, such as Acathla's realm, are implied to resemble the traditional view of Hell; fiery and full of demons looking to torture anything with a soul. Others are more down to Earth, but just as twisted; such as a dimension where people are worked as slaves and deprived any hope of escape. Quor'toth is described as "the darkest of the dark worlds"; although little is known about it, it is hospitable enough for two human beings to survive for several years. An unnamed dimension was ruled over by three powerful creatures known as hell gods; one of these, Glorificus, was deemed too evil by her fellow gods and banished to Earth in mortal form. Another is home to primitive insectoid demons who worshiped the deity known as Jasmine and has an environment deadly to human beings. The demon Sweet came from a presumably musical hell dimension.
Wolfram & Hart Holding Dimension
Although most hell dimensions are described as being chaotic, some have evolved into a relatively structured society. Pylea resembles a medieval Earth full of demon champions and princesses. However, humanoid demons were the dominant species, and actual human beings (referred to as "cows") were treated as slaves, second class citizens and sometimes even food sources. The laws of science are somewhat different in Pylea; vampires are not affected by the world's two suns, but become more aggressive and animalistic than they do on Earth. The world of Arashmahaar is home to many sociable demons such as D'Hoffryn who carry out their evil deeds in a business-like environment. There is also a world populated and named after trolls. The demonic law firm Wolfram & Hart has branches in many various dimensions. They possess their own pocket hell dimension to hold employees who have betrayed the company; this dimension appears as a seemingly-idyllic suburban neighborhood, where residents are forced to relive the same actions over and over again, including torture and mutilation at the hands of a demon living in their basement. When Team Angel brought down the Circle of the Black Thorn, W&H's Senior Partners condemned all of Los Angeles to a hell dimension full of demons. Angel claims that this was only a warning, and that W&H "not only know of places worse than hell, they have timeshares there".
A portal to Quor'toth, the darkest of the dark worlds.
It is not known whether alternate Earth timelines are classified as hell dimensions. Supernatural events can cause reality to branch off in different directions, thus creating alternate universes. However, whereas hell dimensions are entirely different worlds, alternate timelines are merely Earth in a different form. Such worlds include the Wishverse, the Land of Perpetual Wednesday, Crazy Melty Land, and the Land Without Shrimp.
Hell as an afterlife
Despite the belief of many religions that sinners will go to Hell after they die to be punished, they is actually no concrete evidence of this. Humans who do end up in hell dimensions after death usually do so for specific reasons, rather than because it is the natural order of things. The deceased Lilah Morgan claims she has been damned to hell, although this is due to the standard perpetuity clause in her Wolfram & Hart contract, not because of the sins she committed in life. Rather than being subject to torture or punishment in hell, it seems she is simply continuing her employment with the company in another realm. After his physical form was destroyed, the incorporeal Spike begins slipping into a dark and hellish dimension. Although he and Angel believe this was because of the crimes he commited as an evil vampire, it is eventually revealed to be the machinations of Matthias Pavayne, a serial killer who had spent centuries sacrificing souls to hell in order to avoid going there himself. What actually happens to the souls of people who have commited evil is never confirmed, although the poltergeist James Stanley is seen entering a heavenly afterlife after finally being forgiven for killing his lover, suggesting that they are not necessarily damned.