Gotei 13

The Gotei 13 (護廷十三隊, goteijūsantai; 13 Division Imperial Guards, or 13 Court Guard Squads in the English dubbed version) is the organization which most Shinigami join, and one of the three main branches of military in Soul Society. It is split into thirteen divisions (hence the name), with several divisions having certain specializations. The Gotei 13 is subservient to the edicts of the Central 46 Chambers. They also have no jurisdiction over any of the noble families. The symbol of the Gotei 13 is a rhombus, all the 13 Divisions symbols are placed within the rhombus on the back of the Captains haori. The Kido Corps, not officially considered as part of the Gotei 13, is still under the authority of the Central 46, and works together with Soul Society, it is why it is actually here, but officially, both Kido Corps and Omnitsukido are not part of the Gotei 13.

As with any army, the Gotei 13 has ranks. There are 20 ranks for officers but no ranks for non-officers. The lower the number is, the higher the rank. The ranks and their descriptions are below, from highest to lowest in terms of strength.
Commander-General

The Commander-general (総隊長, sōtaichō; Commander-General) is the leader of the Gotei 13 and the Captain of the 1st Division; the implication is that this is an ex officio or a primus inter pares position. The current holder of this title is Shigekuni Yamamoto-Genryūsai.


Captain
The captains of Gotei 13

A Captain (隊長, Taichō; lit. Unit Commander) is the leader of one of the thirteen divisions (with the special case of the Commander-General). A single Gotei 13 "unit" (隊) is estimated at about 200-500 troops.[1]

Captains are generally the most respected shinigami within the Soul Society, with the Central 46 Chambers, Kidō Corps, and Onmitsukidō leaders possibly being on equal status. With one exception, all captains are able to utilize the final stage of their zanpakutō, and are more powerful than even their lieutenants. Knowing bankai gives captains an average of five-to-ten times the strength of shinigami who are unable to perform it.[2] They have also extensively-trained, if not mastered, their bankai, allowing them to actually use the enormous power effectively in battle. Most captains are also skilled in the use of Flash Steps, Kidō, and generally have imperssive knowledge of shinigami history and battle tactics. All of these factors create a huge power-gap between captains and lower-ranked officers.

Gotei 13[bleach.wikia.com]