There are two distinct ‘races’ revealed in Bleach — humans and spirits. Most humans are ordinary, like real-life humans, and cannot see or sense spirits in any way. However, spirits inhabiting artifical human bodies are visible to ordinary humans. Furthermore, about 1 in 50,000 humans can see spirits, and a third of these can see spirits clearly. Some humans, such as the Quincy, have both the power to sense and to fight with spirits. Humans unable to sense spirits can gain the ability to do so by spending enough time around a large source of spirit energy, as evidenced by Chad and Orihime. Spirits have a form composed of reishi (ectoplasm), with an anatomy similar to an ordinary flesh body, including organs and blood. However, this form seems to encompass all of a spirit’s being. In other words, there is no distinction between soul and body.

Plus
A plus (called a whole in the official English version of the manga) is the disembodied spirit of a person who has died. A plus has a spiritual body composed of reishi that resembles its former material body at the time of death. From the chest hangs the Chain of Fatr. This chain ties the soul to something in the world that keeps them there. In the case of a human soul removed from a body that isn’t dead, this chain is tied to the person’s body. If the chain of a spirit tied to an object is broken, the chain begins to corrode. If the corrosion is completed before a shinigami can perform a soul burial on the soul, the plus degrades into a hollow, unless the plus uses shinigami powers to prevent this from occurring.
Pluses are sent to Soul Society by shinigami in a ritual called soul burial.
Once in Soul Society, a spirit is able to live much longer than humans would in the living world, with many spirits aging into the thousands. Spirits generally do not die unless they are murdered, but they can also succumb to diseases, as in the case of Byakuya Kuchiki’s wife, Hisana. Once a spirit dies in Soul Society, its soul is sent back to the living world and is born as a new human. This provides the two worlds with balance.

Shinigami
Shinigami (known as Soul Reapers in the English manga) in Bleach are psychopomps. They are made up of spiritual matter and cannot be seen or detected in any way by normal humans. Their archrivals are the hollows.
The shinigami mainly use soul slayers (zanpakuto) and demon arts (kido), to fight.
Some shinigami have acquired hollow powers using illegal methods; they are known as the vizard.
When captain- or lieutenant-rank shinigami travel to the human world, their power level is limited to 20% of their usual strength to avoid causing unnecessary influence or chaos. They must ask Soul Society for permission to “lift the limit” if they need to release their full power while in the human world.

Modified Soul
Little is known about this experiment authorized by shinigami, except that modified souls were created to hunt hollows by possessing a soul-less human body and supercharging particular aspects of it (strength, speed, etc.). Due to the inhumanity of having dead bodies fight, the shinigami decided to scrap the project, leading to the destruction of all modified souls. Those that were manufactured but yet to possess human bodies were also to be terminated, but some escaped. The only known modified soul in the original manga is Kon. However, in the bount arc of the anime, there are three others: the modified souls Ririn, Claude, and Nova live in gigai created by Kisuke Urahara.

Quincy
The Quincy were a tribe of spiritually aware humans who once fought against the hollows, using weapons composed of spiritual energy to slay them. As opposed to shinigami, who operate using only their inner spiritual energy, Quincy absorb and channel energy from their surroundings. When a shinigami slays a hollow, this cleanses the hollow’s spirit and washes away its sins, allowing it to enter Soul Society. The Quincy technique to slay hollows does not cleanse the spirit, but simply destroys it entirely.
The Quincy’s method of slaying hollows has the propensity to shatter the balance of the universe, because when souls are destroyed, the number of souls entering and leaving Soul Society cannot remain equal. This fact prompted the shinigami to exterminate the Quincy about 200 years before the main storyline. However, in addition to Uryu Ishida, there is at least one other Quincy remaining: Ryuken Ishida, his father.

Hollow
The hollows are the main antagonists of Bleach. They are evil spirits who reside in Hueco Mundo. Like shinigami, hollows are made of spiritual matter, cannot be detected by ordinary humans, and use their internal spiritual power to fight.
All hollows wear masks, except for a few recently created under the direction of former Gotei 13, 5th Division Captain Sosuke Aizen. These hollows are called arrancar and have been able to remove their masks and tap into the powers of the shinigami. Although they carry zanpakuto, the manner in which they release their swords is different from the shinigami, in the sense that rather than modifying the weapon, it modifies the body of the arrancar itself.

Bount (anime only)
The Bounts (baunto, alternately translated “Bount,” “Bounto,” “Baunto,” “Bound,” or “Bountou”) are, like the Quincy, a group of human beings with special abilities. Unlike the Quincy, who destroy hollows, Bounts consume the souls of human beings to survive. They also differ from the Quincy in that eating souls allows them to live for centuries; theoretically, a Bount could live forever. Although the Bounts have a strict rule to consume only the souls of the dead, the last group of Bounts has chosen to begin draining living humans of their souls in order to “power up” their abilities. Each Bount uses a “doll” in combat, a type of familiar possessing its own special abiities. Each doll is unique and is a manifestation of the user’s power, similar to the zanpakuto.