Soul Society (Seireitou)

Soul Society (尸魂界 (ソウル・ソサエティ), Dead Spirit World) is the afterlife. It is meant to correspond to Heaven or Nirvana in Human belief systems. It is the place where most departed souls and the Shinigami dwell.

Thirteen Prefectures
"Did you ever wonder why the Gotei 13 consists of thirteen distinct divisions? Why not simply one or two divisions that handle the protection of the Rukongai and the Human World respectively? Because Soul Society consists of thirteen different districts... or rather, prefectures. Each of them are managed by one of the Gotei 13's divisions and their protection rests on their shoulders. Rukongai happens to be one of these thirteen, run by the First Division."

- Naoya Hōsōkaya speaking to Minato Kuramoto.

Military
The armed forces of Soul Society consist of three independent main branches: the Gotei 13, the Kidō Corps, and the Onmitsukidō.

Gotei 13
The Gotei 13 (護廷十三隊, goteijūsantai; 13 Division Imperial Guards) is the organization which most Shinigami join. It is split into thirteen divisions (hence the name), with several divisions having certain specializations. The 4th Division is the medical/supply division, the 11th Division is the combat division (specialization in swordsmanship), and the 12th Division is the research/scientific division.

The 9th Division is in charge of newspaper and magazine-related duties. It's unclear if the other divisions are specialized, as the four above are the only ones who receive any significant attention apart from their members. Additionally, it should be noted that Suì-Fēng is in charge of both the 2nd Division and the Onmitsukidō. The Gotei 13 somewhat seems to have superiority and little authority over the two branches (i.e., Suì-Fēng, the Commander-in-Chief of the Onmitsukidō and Tessai Tsukabishi, the former Grand Kidō Chief).

Kidō Corps
There is currently little information on the Kidō Corps (鬼道衆, kidōshū), due to the secrecy in which they operate.