The Alliance of Four Great Races was an intergalactic alliance composed of the most advanced races known at that time: the Ancients, the Asgard, the Furlings, and the Nox. They would periodically meet to discuss matters of mutual concern and importance. They also helped to maintain order in the galaxy.
The Asgard used their superior ships to curtail the influence of the Goa'uld. However due to the physical decline of their race and the continued threat of the replicators, they were unable to maintain effective control over the system lords. Towards the end of the Reign of the system Lords the Asgard were merely trading of the treat that the system lords wouldn't dare to challenge them.
The Ancients were the gate builders and seeded many Stargates across both the Milky Way and Pegasus Galaxies. Though it is unknown to what extent, if any, they played in maintaining order in the Milky Way. It is quite possible that they did not have many Starships or City-Ships as they had in the pegusus galaxy due to none of them being found in the milky way.
To ensure universal communication, they created a "universal language" composed of representations of the primary elements.
The Alliance appears to have dissolved after the last remaining Ancients disappeared, and while it is unknown if the other three races still interacted with each other on some level after their passing, it is likely. With the extinction of the Asgard, only the Nox and Furlings remain. However, before their mass suicide, the Asgard proclaimed the Tau'ri as the Fifth Race of the Great Alliance. (SG1: "The Torment of Tantalus", "The Fifth Race", "Unending")
- In Stargate Worlds it would have been revealed, that each species had constructed a potential caretaker species to continue their legacy after they were gone. The Ancients created the humans while the Furlings created the Goa'uld. The Asgard creation, called "Solacris", spectacularly backfired, eventually resulting in the dissolution of the Alliance.