The Andromedans were usually referred to simply as aliens. This is possibly 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 metres in length or height and less than a metre in circumference, with no permanent limbs. They appeared to by 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.
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).
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 600 ships.
The Andromedan invasion fleet was wiped out in the ensuing conflict.
Behind the ScenesEdit
It's popularly believed that Terry Nation originally planned for the shock ending of Series B to be the Federation being invaded by his most famous creations, the Daleks. However Chris Boucher recalls this only as a tentative suggestion that he and the producer 'stamped on very firmly', with the Andromedans replacing the Daleks.