Andromedans is the name widely used for the alien species, native to another galaxy, which provoked the Intergalactic War.
The Andromedans were usually referred to simply as aliens. This is possible because Andromeda includes stars from Earth's own galaxy as well as the distant galaxy M31 from which they presumably originated.
The Andromedans seen were approximately two meters in length or height and less than a meter in circumference, with no permanent limbs. They appeared to be amorphous in form and were able to assume the appearance of specific human individuals, although maintaining such a shape required effort. They reverted to their original form upon death.
The Andromedans were capable of intergalactic travel which appeared to be beyond the Federation's technology. Presumably they had developed a form of intergalactic drive.
While this would seem to suggest their technology was greatly superior, the Liberator was able to stand off their invasion force unassisted for at least an hour without being critically damaged (although the aliens were not able to bring their entire force to bear due to the antimatter minefield still being largely in place.
The Federation was aware of the threat of possible extragalactic invasion and had constructed an antimatter minefield beyond the rim of the galaxy near Star One.
By unknown means a small group of Andromedans was able to penetrate the minefield and infiltrate Star One, replacing the human staff of the installation. With the aid of Travis they planned to shut down the minefield and invade the Federation with a fleet of of 600 ships.
Blake was able to stop the complete shutdown of the mines and the Federation was warned of the invading fleet, resulting in the Intergalactic War.
The Andromedan invasion fleet was wiped out in the ensuing conflict.