Eberron is the name for the land of the world, and is also referred to as the Dragon Between. Siberys, the Dragon Above, is the name given to the planetary rings which surround the planet. Khyber, the Dragon Below, is the name given to the underworld, and is similar to the Underdark in many other settings. According to the creation story, the world was formed when the progenitor wyrms changed their form into what they are now. Siberys created the dragons, Eberron created humanoids and other "lower races", and Khyber created the "demons" of the world.
It was one of the 16 original released servers. Many other servers have since been merged into Khyber.