Captains Lounge

Basically, Shinigami are departed human souls that have gained supernatural powers. As such, their bodies are composed of spiritrons (spirit particles) 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. Additionally, they can be injured and die like regular humans, though it takes considerably greater injuries for the latter to occur than it would in regular humans. Additionally, unlike humans, they do not have to eat anything to sustain themselves. Shinigami and other spiritual beings (Pluses and dwellers of Soul Society) only become hungry when they used too much Spiritual Energy within a short amount of time.

The most prominent supernatural power possessed by a Shinigami is their zanpakutō, a supernatural sword generated from the Shinigami's soul. Shinigami also naturally give off spiritual energy, which varies greatly in magnitude with the strength of a Shinigami. A zanpakutō is a physical manifestation of this force concentrated into a blade.[1]

Shinigami operations are based out of 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. In addition, it is a crime for a Shinigami to remain in the human world longer than a certain time limit.