In Japanese mythology, a shinigami is a spirit of the dead. Some Japanese authors and manga artists adapt the legend into their series.
In the manga/anime series Bleach, Shinigami (Soul Reapers in the English dub) are the protectors of souls. Their duties are (1) to guide souls that do not cross over to the Soul Society, and (2) to cleanse Hollows, monsters created when normal souls become corrupted by their regrets. They are spirits and cannot be seen by ordinary humans.
Death Note Edit
In the manga/anime series Death Note, shinigami are known as the gods of death. They can kill any human just by writing his name in their notebooks, which are called death notes. In this way, they gain the life they steal from the humans. For example, if you are meant to live until you're 64 and a shinigami kills you when you are 42, the remaining 22 years are added to that shinigami's lifespan. Their eyes, known as shinigami eyes, can see a human's lifespan and name floating above his head. They live in their own world and don't usually go down to Earth, unless they let their notebooks fall in it. They can be seen by humans who have touched their notebooks and nobody else.
Yami no Matsuei Edit
The manga/anime series Yami no Matsuei (released in English as Descendants of Darkness) centers around the adventures of a shinigami department. They are charged with ensuring the proper passage of souls into death as inscribed in the Book of the Dead. When anomalies occur, they may interfere in the daily lives of humans and be seen by them.