Mass Effect, Mass Effect 2 & Mass Effect 3
The Council is an executive committee composed of representatives from the Asari Republic, the Turian Hierarchy, and the Salarian Union. Though they have no official power over the independent governments of other species, the Council's decisions carry great weight throughout the galaxy. No single Council race is strong enough to defy the other two, and all have a vested interest in compromise and cooperation.
Each of the Council species has general characteristics associated with the various aspects of governing the galaxy. The asari are typically seen as diplomats and mediators. The salarians gather intelligence and information. The turians provide the bulk of the military and peacekeeping forces.
Any species granted an embassy on the Citadel is considered an associate member, bound by the accords of the Citadel Conventions. Associate members may bring issues to the attention of the Council, though they have no input on the decision. The human Systems Alliance became an associate member of the Citadel in 2165.