Adonar is the High Plane, and the home plane of the Elves.
All Elves (with the exception of the mixed-blood Bair) were born there, as Elves cannot reproduce outside of their home plane. It is considered a peaceful and civilized place, and lacks the passionate vibrancy that some Elves find attractive in Mithgar. There are several places where Adonar can connect to Mithgar, and thus allows for easy travel between planes.
Adonar does not seem to have any oceans, since Aravan had never seen one until he came to Mithgar.
Few other races seem to have traveled to Adonar, even before the bloodways restricted planar travel. When Adon chose to sever travel between the planes, in order to restrict the ability of the Foul Folk to invade Mithgar and Adonar, travel to Adonar became restricted to Elves until Bair convinced him to lift the Ban. It also became a one-way journey; once an Elf had returned to Adonar, he could not visit Mithgar again.
Adonar is also the dwelling place of the gods other than Gyphon, who are approachable by any who are able to travel there. The exact location of the gods is unknown, but personal meetings with them seem to be a rarity.