Shinigami are death gods, a recurring concept in multiple anime, notably in Bleach and Death Note
Shinigami (死神) literally refers to the concept of a "death god." In Japan this god is similar in concept to the European Grim Reaper, leading the term to sometimes be translated as "reaper." However, as with most Japanese lore, there is usually no one single shinigami, but rather a variety of them, all leading souls to the land of the dead.
However, because so many different series utilize the concept of death gods, how they function (and why) varies greatly. Here are some of the most popular anime shinigami types:
In Death Note, shinigami carry the eponymous death notes, which they use to kill humans and lengthen their own lives. This is in contrast to many shinigami in other series, who are in place to protect humans from dying before they're intended and/or to guide their souls to a place of final rest. In Death Note, any shinigami who extends the lifespan of a human dies.
An entire army of shinigami exist in Bleach, where they protect humans from creatures known as " hollows" and direct them to Soul Society after their deaths. Shinigami in this series can come in the form of a variety of military rankings which generally denote their overall strength both physically and spiritually. The highest-ranked are known as "captains," with "lieutenants" and those of numbered seats following in consecutively lessened rank. In addition, the shinigami here are divided into thirteen different squads, each with their own purpose. Some, for example, specialize in medicine and hospitalization, whereas others focus on research, reconaissance, and battling.
In Supernatural we meet Grim Reaper Tessa who is after Dean. She first appears in her true form when Dean flat-lines, but Dean, having an out-of-body experience, manages to chase her away and thus his hearts starts beating once more. He shortly meets her again, this time in the form of a young woman who resembles his mother. She tries to convince Dean to come with her and cross over but Dean wants to keep fighting. Sam and Dean learn that Grim Reapers can take on any form. Dean confronts Tessa and she says that if he stays on the mortal plane he'll become an angry spirit.