(information courtesy of the Bleach manga)
Shinigami are unearthly beings living in a world ruled by a different logic than the World of the Living. They possess high Reiryoku and a body of Reishi. They are invisible to those without spiritual powers.
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 potential Shinigami is born among the residents of the Rukongai, and in some rare cases from Seireitei. Shinigami receive salaries, like workers in the Human World and also earn bounties for defeating Hollows.
All Shinigami have vents at their wrists which release their Reiatsu. If they are blocked off, the Shinigami in question would be incinerated by their own Reiatsu from the inside out. If a Shinigami's Saketsu (Chain Binding) and Hakusui (Soul Sleep) are pierced, their spiritual powers are sealed up and they most likely can never regain their lost powers again.
Shinigami have a number of supernatural abilities:
Longevity: Shinigami do not visibly age at the same rate as Humans and tend to maintain a youthful appearance for centuries. Shinigami can live for much longer than Humans, with some Shinigami, such as Retsu Unohana, being over 1,000 years old and Genryūsai Shigekuni Yamamoto being at least 2,100 years old.
Enhanced Durability: While Shinigami can be injured and killed like regular Humans, it takes considerably greater injuries to kill a Shinigami than it would to kill Humans. Decapitation is the only guaranteed method of killing a Shinigami.
Reiryoku: Shinigami naturally possess high levels of Spiritual Power, which varies greatly in magnitude with the strength of a Shinigami. The more power a Shinigami has, the sharper their movements will be. A consequence of having Spiritual Power is that unlike powerless spirits, Shinigami need to eat.
Zanpakutō: A Shinigami weapon, its shape is typically similar to a Japanese katana. Low ranking officers are issued an Asauchi sword, but high-ranking officers arm themselves with unique swords generated from their own souls. 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. Its shape differs depending on its owner.
Exiled Shinigami:
There are some Shinigami who exist in exile, namely Yoruichi Shihōin, Tessai Tsukabishi, and Kisuke Urahara. They will work with the Shinigami at times, but still exist in exile.
