Founded: c. 500 BCE (c. 2,700 years old)
[Mass Effect Wiki]
The Citadel Council is an executive committee of ambassadors who resolve disputes of interstellar significance and pass judgement on violations of galactic law. The Council consists of one representative from each species granted full membership to the Council. As of ME3, only four species have been granted full membership: the asari, the salarians, the turians, and the humans. Full membership is understood to entail a number of responsibilities, such as a commitment to the military defense of Citadel Space, the rendering of economic aid in times of disaster, and the ability to independently care for its own citizens.
A number of species hold associate membership with the Council, such as the volus, the elcor, and the hanar. An associate membership for a species grants that species an embassy on the Citadel, the right to petition the Council with matters of interstellar significance, and the military protection of the Council Fleet.
Citadel Security Services (C-Sec)
[Mass Effect Wiki]
C-Sec is the police force which maintains law and order on the Citadel. It is headed by an Executor and answers to the Council. It uses a quasi-military rank structure and membership is voluntary. Although its members hail from all Council species, turians and humans make up the bulk of the membership.
Citadel Office of Special Tactics and Reconnaissance (Spectres)
Founded: 693 CE (1,490 years old in ME1)
[Mass Effect Wiki]
The Spectres are an association of law enforcement and covert intelligence agents, individually charged by the Council with protecting galactic peace and stability. The Spectres have no command structure; agents act independently, or choose to cooperate in small groups. In Citadel Space, a Spectre’s actions are held to be above the law: Spectres have been known to endanger or even kill civilians in order to stop a greater threat. The only check on their power: the Council can revoke what they have given, and have been known to do so in cases of gross misconduct. In these rare cases, another Spectre is usually assigned to hunt down and kill their disgraced peer.
In tense situations, the Council occasionally sends Spectres outside of Citadel Space, where they may act as representatives of Council interests. Assignment of a Spectre is less contentious than a military deployment, but still sends a clear message that the Council is watching events with interest.
Special Tasks Group (STG)
The STG is a branch of the salarian military, devoted to espionage and covert actions. The STG was the inspiration for the Spectres. Even so, the Council still makes use of the skills of the STG today: STG units are preferred in some situations, as the STG tends to attract less attention than a Spectre would.
Systems Alliance Navy (Alliance)
The Systems Alliance Navy, or Alliance for short, is the major military force of the Systems Alliance, the unified government that represents Earth and its colonies in galactic matters. The Alliance Navy is notable for being one of the greater military forces in Citadel Space, despite being fielded by one of the most junior spacefaring races in the galaxy and despite service being entirely voluntary.
N7 is an Alliance MVC: “N” is the vocational code for “special forces” and “7” is the proficiency indicator for “highest level of proficiency”. Thus, the N7s are the most elite spec-ops soldiers to come out of the Alliance’s training program.
Cerberus is a privately funded pro-human covert research and paramilitary group. Founded by the Illusive Man, Cerberus is rumored to have started as an Alliance black-ops unit gone rogue. It is funded under the table by various billionare industrialists who believe in its mission, as well as a number of front businesses in the military-industrial complex. Outsiders consider its experiments dangerous and reckless, its activities as terrorist, and its agenda as human-supremacist.
Cerberus is organized into cells. For example, the project which brought Commander Shepard back to life was Lazarus Cell, led by Operative Miranda Lawson. Each cell operates independently of the others, reporting only to the Illusive Man himself and having no knowledge of the activities in other cells. A Cerberus cell may have a scientific, political, or military objective.
The Blue Suns are a “private security company” (read: mercenary organization) with operations spread throughout the Terminus Systems. The organization was jointly founded by Vido Santiago and Zaeed Massani, an arrangement that came to a head when Vido betrayed Zaeed and left him for dead.
Eclipse is a mercenary organization active in the Terminus Systems. Membership is largely composed of asari and salarians, with some humans. Compared to other merc organizations, Eclipse has a notable reliance on assault mechs, techs, and biotics.
The Blood Pack is a mercenary organization active in the Terminus Systems. The leadership is krogan, the infantry and heavy weapons support are vorcha, and they are often accompanied by trained attack varren.