They are souls with a high level of innate Spiritual Energy, recruited from the ranks of the residents and nobility of Soul Society. Their bodies are composed of Reishi instead of atoms. This means that just like most other types of spirits, they can only be seen by other spiritually aware entities, which excludes most Humans. Nevertheless, they are quite capable of influencing their environment. When souls with exceptional spiritual energy train their bodies, they reach the level of Shinigami. The most talented of them become affiliated with various organizations like the Gotei 13 and the Onmitsukidō. Sometimes, a Shinigami is born among the residents of the Rukongai, and in some rare cases from Seireitei. Shinigami are the personification of death. Their job is to send spirits to the Soul Society (the afterlife) in order to maintain a balance of souls between it and the material world, and to exorcise evil spirits. Because of their duties Shinigami possesses supernatural abilities such as:

Racial Abilities:

Longevity: While they can age and die of natural causes or as a result of physical injury as regular humans, Shinigami have extended life-spans and it is not uncommon for one to live for thousands of years.
Enhanced Endurance: While they can be injured and die like regular humans, it takes considerably greater injuries for the latter to occur than it would in regular humans.

Reiryoku: Shinigami also naturally give off Spiritual Energy, which varies greatly in magnitude with the strength of a Shinigami. Additionally, unlike humans, they do not have to eat anything to sustain themselves, though they engage in the act none the less as Shinigami and other spiritual beings only become hungry when they used too much Spiritual Energy within a short amount of time.

Kidō: The technique that Shinigami use to focus their Spiritual Energy into magic spells of various powers.

Hohō: A variety of techniques associated with movement, including jumping and shunpo (flash step).

Zanpakutō: The most prominent supernatural power possessed by a Shinigami is their Zanpakutō, a supernatural sword (which can be in the form of any kind of Japanese sword) generated from the Shinigami's soul. A Zanpakutō is a physical manifestation of this force concentrated into a blade. All Shinigami have a Zanpakutō, but not all of them carry one with them. Shinigami operations are based out of the Soul Society, where departed human souls reside. Travel between the Human World and Soul Society is extremely limited and monitored, but some Shinigami are stationed in the human world to carry out their duties and therefore must often travel between the two. It is also implied that Shinigami receive salaries, like workers in the real world.

Appearance:

Most Shinigami appear like Humans, a notable difference being their mostly old-fashioned garments by 20th-21st century standards (see Gotei 13 Uniforms and Onmitsukidō Uniforms), and their Zanpakutō which is in most cases tied to their obi sash.

There are, however, a few Shinigami who are less Human-like in appearance. For example it seems that between Shinigami there are much greater variations in body-sizes than between Humans, as there are real giant-like entities (e. g. the four guards of the four gates to Seireitei) and ones with extremely small stature among them (e. g. Yachiru Kusajishi). This variation seems to occur not just between Shinigami, but also between regular dwellers of Soul Society, too. Variations do not just occur in size but in overall appearance, too, the most notable example being Captain Sajin Komamura, who appears like an anthropomorphic canine.

Duties:

Konsō (魂葬, Soul Burial): the process by which a Shinigami sends wandering Pluses (souls) in the living world and send them to their respective place. Either Soul Society if they are good in life, or to Hell if their life was full of evil acts (such as murders, theft, etc). Konsō is performed by using the hilt of the Shinigami's Zanpakutō, tapping the soul on the head and transporting them to the afterlife.

Hollow Cleansing: When a Shinigami slays a Hollow (evil spirit) with their Zanpakutō, they cleanse their sins. Once this takes place, a Hollow will return to its former state as a Plus and is led into Soul Society. This is called sublimation. Souls that live in Soul Society are reborn again into the human world. Not all Hollows are sent to the Soul Society. Since Zanpakutō are only capable of cleansing sins committed after becoming a Hollow, Hollows that committed serious crimes when they were still Human are sent to Hell when they are slain. Also, if they are destroyed by a Quincy, Hollows are completely vanquished and cannot ever return to the Soul Society.

Soul Governance: Shinigami are responsible for governing the flow of spirits between the Human World and Soul Society. This is commonly carried out by balancing the total number of souls by moving and managing them in the human world. All of the soul-related jobs are under their authority.