The Goteijusantai is the oldest, and only, collection of Shinigami in any dimension. Shinigami are trained by the Shinou Academy, and placed into a division, in a bit of a military-like structure, and exist to oversee the flow of souls from one world to the next, as well as protect living humans from Hollows. The Goteijusantai has, officially, thirteen divisions, with two somewhat-independent off-shoots; the Onmitsukidou, or Special Operations Force; and the Kidoushu, or the Kidou Corps, which specialises in the research and teaching of Kidou magic. As of now, the Onmitsukidou and the Nibantai, or Second Division, are led by the same Captain and Lieutenant, effectively marrying the two together.
The Ichibantai are the enforcement division. At one time, they enforced the laws set down by the Central 46, but with the Central wiped out, the Sotaichou has opted to replace Central, the Seireitei's government body, with a council comprised of the divisions' captains, called the Council of Thirteen. The captain of the first division, the Sotaichou, leads the entire Goteijusantai.
The Nibantai have been married to the Onmitsukidou, or the Special Operations, for so long, that no one really remembers what they did before this arrangement. As such, the Nibantai is generally integrated with the Onmitsukidou, and the Captain of the Nibantai is also the Commander of the Special Operations Forces. They handle very sensitive and important message delivery, dangerous criminal handling and detainment, and at one time were under direct command of the Central 46; this is now the Council of Thirteen.
The only division to focus entirely on it, but not the only one to handle it overall, the Sanbantai are entirely focused on living world defence. Most Goteijusantai patrols in the living world are from Sanbantai, and they take two weeks out, one week in the office, and one week off entirely. The schedule is a bit rigorous, and doesn't leave much room for a social or family life, so those that join Sanbantai tend to have neither.
The Fourth Division are primarily healers; from intensive care, to general wellness, to field healing and emergency services, the Fourth Division handles it all. Oh, they also keep your office stocked with paper and staplers. There are several different, focused units within the Fourth Division, which handle specific things, such as heart-related problems, an emergency ward, an intensive care ward, and so on.
The Gobantai are messengers, running information and paperwork between one division and another. The Fifth Division may also on occasion run out to Rukongai or the living world, and generally spend a lot of their time doing something.
Traditionally led by the noble Jeon family, the Sixth Division houses, tracks, and maintains the Great Archives beneath the Seireitei. Every important event that happens within the Seireitei, from Hollow attacks, to emergency living world situations, to crimes, births, marriages, and deaths, are recorded and stored in the Great Archives.
With so many different divisions, inter-division relations are extremely important and required to be maintained for the Goteijusantai to continue operating as a single unit. As such, the Nanabantai operate as the bridge between divisions, helping improve strained relationships, both between two divisions and between two officers, and keep an eye out for treason and rebellion.
While, in most instances, it is the Goteijusantai bringing the fight to their enemies, in situations where the enemy brings the fight to the Goteijusantai, the Hachibantai are the first to react. They handle both the Seireitei's defence from threats, real or perceived, and security patrols around the Seireitei and the first few districts of Rukongai.
When something goes amiss or awry, and no one seems to have any answers, it is the Kyubantai that answers that cry for help; the investigation division looks into crimes, trouble, and other sorts of dangerous situations and events, in order to find the truth and better inform the Seireitei how to approach and handle any given situation. Unfortunately, this means they are the first in unknown situations, and often take the brunt of lacking knowledge.
All news and general goings-on in the Seireitei are often known by the Tenth Division first; while they do handle the arrangement and publication of the Seireitei news magazine, the Seireitei Communication, they are also the reconnaissance team, and are sent out to gather intel about enemies and opposing groups before the rest of the Seireitei faces them. Much like Kyubantai, Jubantai can end up very quickly in rather dangerous situations.
As the melee combat division, the Eleventh Division essentially spends their time honing and perfecting their Zanjutsu skill, and are often found sparring one another. The division is usually the first on the front-lines, perhaps after eighth, ninth, and tenth, but they are also often the last ones off the battlefield. The division used to be quite unruly, but these days, the new Captain's managed to calm their battle-thirst a bit.
Where the Fourth Division often deals in medical research, the Twelfth deals in scientific research, and develop new technologies, and improve on existing ones, in order to make the lives of the Goteijusantai, and sometimes the Seireitei as a whole, much easier. Sometimes they can be a bit sketchy with their morality and sense of ethics, but none can deny that their inventions are quite the impressive improvements.
The Thirteenth Division are responsible for maintaining and monitoring the Hollow sensors in the human world, as well as keeping the communication lines between the Soul Society and the living world up and stable. They also often react first to a Hollow sensor going off, and tend to act as backup for the Third Division in the living world.
The Fourth Division deals in medical research, the Twelfth in scientific, and the Kidou Corps, or Kidoshu, in Kidou research. The most masterful of Kidou users often end up in Kidoshu, helping expand on the Goteijusantai's understanding of the Kidou arts. They also assist Twelfth Division with some Kidou-related technologies, and help teach Kidou classes at the Shino Academy.
Some individuals prefer to remain separate from the squabbles of others, and mind their own business. Alternatively, perhaps they were scorned by too many groups, or just never did feel any attachment to anyone else, anyway. These individuals may or may not be allied with something when push comes to shove, but officially, they're solo.
The Shinkuchou are one of the organised groups in Rukongai. As the Goteijusantai does not pay much attention to the higher-numbered districts, the Shinkuchou are one of the groups that formed in effort of keeping some semblance of control over the outer districts. In some sense, this was successful, but depending upon whom, exactly, is calling the shots currently, the Shinkuchou can and do see plenty of their own internal strife and corruption. The Butterflies rule primarily in Eastern Rukongai, districts 45-74.
The Tetsuryuu are one of the organised groups in Rukongai. As the Goteijusantai does not pay much attention to the higher-numbered districts, the Tetsuryuu are one of the groups that formed in effort of keeping some semblance of control over the outer districts. In some sense, this was successful, but depending upon whom, exactly, is calling the shots currently, the Tetsuryuu can and do see plenty of their own internal strife and corruption. The Dragons rule primarily in Western Rukongai, districts 42-68.