The world of BLEACH and it’s inhabitants
The...er...shorter version
The world in which Hiyori comes from is made up of three different planes of existence (that we are made aware of).
There is the living realm, where all that are, well, living live. This would be where all the humans and any other living creature resides.
The second realm is the realm of (good) spirits, Soul Society. This is where the Shinigami exist. This is also where all souls go to live out their afterlife and be reincarnated. The Shinigami are spirits that protect the souls of the dead and help them move on when they are having trouble. They also protect the living and the dead from evil spirits known as Hollow. Everything in this realm is made up of a high concentration of reiatsu (spirit energy). The rules are very strict here and all those with high enough spirit energy often join the Shinigami Academy.
While this place is the afterlife not everything is great. While there is a hell for those that are truly horrible Soul Society can be a hell depending upon where the soul ends up after they move on. Families that die together (or not even that far apart) are often split up. Some of them ending up in the more hellish parts of the afterlife. There is no say in where one ends up, it is all up to chance. The area around the Gotei 13 (the central guard (made up of 13 divisions) court where nobles (though there aren’t many of those left now days) and Shinigami reside) expands in a circle. This area is known as the Rukongai. The inner rings are nicer areas with less crime but the further out you go the worser things become. The most outer rings are the worst, only the strong survive there.
A note, only those with high spiritual energy require food to survive. Although people will still eat things they enjoy just because they can.
The last realm, which lie between the realm of the living and the realm of spirits, is Hueco Mundo. Hueco Mundo is a barren place, devoid of nearly all life. The only things that can live in Hueco Mundo are the souls of the damned, the Hollow. Hollow are the souls of those there either did something evil in their living life, those that couldn’t move on after death, or the spirits of those that were attacked by Hollow but not eaten. This realm is also made up of a high concentration of reiatsu. But it is not enough to sustain Hollow so they often slip into the living or spirit realm to feed.
This realm is nothing more than a large desert where Hollow run free. This is also where Las Noches is located. This is Aien’s fortress where all the Arrancar reside.
The barriers between these realm are thin but only those with the know-how can travel through them. Shinigami use special gates and are accompanied by Jigoku Chou (hell butterflies that act as messengers/protectors) so that they are not eaten by their tunnels defense (a giant steam engine that squashes anything that gets in it’s way). The Hollow (and later Arrancar) use a similar method but it is often more noticeable (to those with high spiritual powers), as they often tear through the veil.
Shinigami:
As stated before Shinigami are the protectors of both the living and the dead. They help the dead move on when they have trouble, ferry the dead to the afterlife, and fight off/kill (destroying their mask kills them) Hollow. Killing a Hollow purifies the evil spirit and reverts it back to it’s once good nature. That is if the Hollow was a good spirit to begin with. If not, when the Hollow is killed the gates of hell appear and open, dragging the evil spirit into hell. Only those that preformed the most horrific of deeds in life are taken to hell. All murders done after becoming Hollow are automatically pardoned if the Hollow was a good spirit in the beginning.
Shinigami must all attend the Shinigami Academy before they can join the Gotei 13. There they learn to hone their spiritual energies. Learning kidou (demon magic), hohou (increased speed, floating in air and increased jumping ability), and Zanpakutou (their swords, which have a spirit of their own). Once they have graduated they are moved into the Gotei 13 as a seated or unseated officer.
All Shinigami carry a Zanpakutou. This sword is connected to the Shinigami and has a living spirit of it’s own. Each one has a different name and varying abilities that fall under one of the elements. Only those that are in tune with their Zanpakutou can master it. There are three stages of the Zanpakuto. First is it’s sealed state, where it is a regular sword. Second is the initial release, or Shikai. This is when the powers of the Zanpakutou are activated, often the state of the sword changes, sometimes becoming something other than a sword. Lastly there is the Bankai. This is when the Zanpakutou has been completely mastered and the wielder is one with their sword’s spirit. Bankai very as much as Shikai but are often larger and flashier.
Hollow:
As stated before these are the corrupted spirits of the dead. Hollow are usually large (although sometimes they can be smaller in size), they are usually demonic in appearance, they all have special abilities that vary, and all wear a mask of bone over their faces (effectively hiding the soul that they once were).
Above normal Hollow there are four levels.
Menos are simply groups of Hollow that have combined into one entity. These Hollow are drawn to each other, feeding on one another until they combine enough to form a Gillian.
Gillian (aka Meno Grande) are the weakest of Menos and the first stage in evolution. They all look the same. They are large, black and all have the same mask upon their face. While they have no personality of their own and simply go on instinct and are generally weak. This makes them incredibly dangerous, especially en masse.
Adjuchas are what become of a Gillian after they have devoured enough of their own kind. Like general Hollow they vary in size, shape and ability. But they are all not much bigger than the average human. Like Gillian they must feed on other Hollow, otherwise they will revert to a Gillian and never be able to return to an Adjuchas again.
Vasto Lorde are the strongest of all Hollow. They are humanoid in appearance. Very little is known about Vasto Lorde due to the fact that it is nearly impossible for an Adjuchas to evolve to that level.
Lastly there are the Arrancar. They are a final evolved form of a Hollow. Either through natural means (devouring enough of their own kind and breaking their mask themselves) or through the use of Hougyoku (a very powerful object created by Urahara, that Aizen now has in his possession and was once hidden within Rukia‘s gigai). Many Arrancar were created using the Hougyoku.
Once becoming an Arrancar they Hollow appears to be completely human aside from the remnants of their mask. They gain the abilities of a Sinigami, meaning they now have a Zanpakutou of their own. Just like Shinigami they Zanpakutou have a “Shikai” (although it would appear to be more of Bankai state). The Arrancar call this their Resurrecion state. When their Zanpakutou is released they revert back to a state similar to that of their original Hollow form. Unlike Shinigami an Arrancar’s Zanpakutou does not have a spirit of its own.
Vizard:
Vizard, like Arrancar, don’t fall under classification of a Shinigami or a Hollow. Vizard are the antithesis (the complete opposite) of the Arrancar. They were once Shinigami that, for one reason or another, gained the abilities of a Hollow. As far as anyone knows there aren’t that many of them. Just the ones created by Aizen (Hiyori and the others) and then Ichigo (accidentally, or maybe not - you never know with that man, created by Urahara).
Vizard appear entirely human, there is no way to tell what lies beneath. Unlike Arrancar they have a dual personality that must always be kept in check. This is the Hollow (combined with their Zanpakutou it must be pretty crowded in their heads). Their Shinigami abilities are enhanced/combined with their Hollow’s abilities when they don their masks. Making them stronger than any Shinigami. Unfortunately training has only allowed them to keep their masks/enhanced powers activated for a period of three minuets. After that they only have their Shinigami abilities to rely on. Mashiro is an exception to this. Unlike the other’s she was perfectly in tune with her Hollow and required no training from the very beginning. She can keep her mask on for well over an hour.
Since they are a fused with what the Shinigami consider to be their enemies they are rogues. They are no longer allowed into Soul Society and must hide themselves in the living realm.