The Citadel Council is composed of a representative from each member species. Although officially the Council has no power over independent governments of other species, the decisions they make can have a major impact on the rest of the galaxy.
Each Council species has general characteristics associated with the various aspects of governing the galaxy. The Asari, the oldest Council species, are seen as diplomats and mediators. The Salarian, with the help for there legendary Special Task Group agents, provide intelligence and information. With the largest organized military, the Turian make up the bulk of the military and peacekeeping forces.
A species that is granted an embassy of the Citadel is considered an associate member. Associate member may bring issues to the Council's attention, they however don't have an impact on the final decision.