The Interstellar Alliance is an economic and political force which formed after the Shadow War ended (near the end of 2261). The Interstellar Alliance includes many of the Non-Aligned Worlds (Minor) and the major powers including the Centauri Republic, Earth Alliance, Minbari Federation, and Narn Regime.
The government is parliamentary style government with the 'lower house' being made up of the former Non-Aligned Worlds (Minor) and the upper house made up of the major powers (Centauri Republic, Earth Alliance, Minbari Federation, and Narn Regime). The upper house selects a president who serves until he or she is removed by the parlament. The lower house approves the person selected by the upper house.
It should be noted that the member worlds retain their own governments and leaders, such as the Earth president and the Centauri emperor, and the Interstellar Alliance president is not in a position of direct command over their internal affairs. The president acts, in many ways, as chief administrator who organizes and manages the various elements of the organization. In the larger context, the ISA acts as an umbrella under which various members can collaborate on various issues such as they relate to diplomacy, trade, exploration, humanitarian aid, and defending members and those who may ask for assistance from hostile powers and piracy.