The Alliance, or, to give it its full title "The Alliance of the Twin Worlds" is a politicial, economic and military allaince between the native species of Old Earth (humans) and Paplin (the B'ra'Sha)
The two species met seven hundred years before the birth of the Alliance as both species were searching for habitable worlds. From that point they got closer and closer until the First Ra'Shain War erupted and the humans sided with the B'ra'Sha aginast the militant cousins, the Ra'Shain. Their joint victory against the Ra'Shain led to the formal declaration of the Alliance.
The central star systems of the Alliance are known as the Core Worlds, and the central world, where the humans and B'ra'Sha met and the President, Senate and Star Command HQ are all based is on Orsimo.