A shinigami is a death god, as per Japanese culture, and can be seen in several modern anime/manga stories such as Bleach, Death Note, and Full Moon wo Sagashite.
Bleach and Shinigami in #suburbansenshi2
Soul Society's concern is with the dead and the cycle of life. As such, the few shinigami who regularly visit the Ten'ou House had some connection with the place in life. It is the policy of Soul Society, and thus of shinigami, to stay out of mortal affairs; while they may enjoy the occasional vacation on Earth, when a shinigami returns to Soul Society, mortals without unusual spiritual power simply forget that the shinigami ever existed. This fact makes maintaining relationships with most Humans a difficult process at best.
The shinigami's weapon of choice is the zanpakuto, and one's ability to unlock the powers of their zanpakuto is generally the best measure of their power, though exceptions can and do exist. Many characters have died and taken on shinigami status, including Sakura Xadium Aino and Furu Ikari. Both have returned to some semblance of life, though Sakura retains much of the knowledge she gained during her stay there.