Shinigami are gods of death.

Shinigami live in the Shinigami Realm and can use their Death Notes to kill humans. This is an easy task for them because of the Shinigami Eyes.

Even if stabbed in the heart or shot in the head, Shinigami cannot die. This is because they can add years to their life spans by killing humans before they are fated to die. They can only be seen by humans who have touched their death note. These facts and many more important ones about shinigami can be found in the instructions Ryuk wrote for the Death Note.