The Shinigami, also known as Death Gods or Gods of Death, are the overarching antagonists of the entire Death Note franchise. The Shinigami are an extra-dimensional species within the Death Note universe who survive by having humans killed to extend their own lifespans, granting them practical immortality. They accomplish this by passing down the titular Death Note into the human world and allowing it to fall into the hands of a random human in hopes that they will use it to kill other people with Light Yagami being the most notable example.
The Shinigami have varied appearances with their physical appearances being made of elements deemed unnatural in the human world. Since each Shinigami posses their own Death Notes, they are able to carry them around and use both them and a pen despite some of them lacking any limbs (hands, feet, etc.). Shinigami also posses wings which are visible when used in flight, especially when flying in the Human World, but will retract when not in use.
Nature and Abilities
The Shinigami themselves are very intelligent creatures to the point that they each posses their own written languages that they can understand even if they don't write or speak it. The series creator, Takeshi Obata, also said that the Shinigami can understand Earthly languages as well such as Ryuk writing a copy of the Death Note in English. Each Shinigami is assigned a job, but the amount of work they have to do is never stated and their true job is to find humans and make them die earlier to extend their overall lifespans. Shinigami are tasked with taking life because they are death gods, but they do have the ability to save lives if need be, but this will cost them their own life in the process.
Beyond monitoring the actions of the human world, the Shinigami live a relatively meaningless and boring existence to the point that they don't even know why they exist and what they're supposed to do.
The only reason they maintain their immortality is that they are afraid to die and will simply perish and dissolve into mere dust upon death, so they send out Death Notes to make sure that humans are killed and their lifespan is constantly being extended. Because of this, Shinigami normally live forever. The Shinigami themselves can be male or female, but are forbidden to have sexual relations with both other Shinigami and Humans in general.
They do not reproduce and the Death Note keeps them alive and prevents the species from suffering extinction. Despite this however, Shinigami can have feelings towards the opposite sex such as when Ryuk was embarrassed by Misa Amane when she hugged him. Humans can also detect what gender a Shinigami is and the Shinigami can detect their own genders as well. Other powers possessed by the Shinigami include the ability to materialize and de-materialize their bodies at will and can walk through walls. Shinigami are also invisible to virtually all humans and can only be seen by any human that touches a Death Note. Physical attacks such as guns will also have no affect on them at all.