Name 'Arborea' is not used in-game. Devs have stated in an interview that they found it "confusing". Name refers to trees (from Latin arbor, "tree"), and they found it confusing, since Arborea has many other biotopes, like huge deserts and snowfields. Name is still widely know. (It should be noted that when running game is listed as "The Exiled Realm of Arborea".)
There might be another explanation to name, the huge World Trees, one of which is shown in introductory cutscene on Stepstone Isle. There are few other trees infused with Divine essence, on Island of Dawn for example.
The realm of Arborea is comprised of four major continents. In the southern east is Southern Arun, in the northern east is Northern Arun, in the southern west is Southern Shara, and in the northern west is Northern Shara.
Although there are a minority of boats in multiple locations such as Velika Outskirts, civilization prefers to use pegasi for quick travel between provinces. Mounts have been a major choice of travel. Most used mounts are horses due to their high stamina.
When Arun and Shara began their dream, their intention was to create a peaceful realm with no war to ever touch it. That intention continued for an unknown time, only to eventually be attacked by the Argons from a different realm.
The Argons war
Far from their outlandish homeworld, the Argons have decided to invade the realm of Arborea for an unknown motive. They took the existing living of Arborea by surprise and did not expect such as it was their first war.  The Argons are mainly attacking Northern Shara, but have been spotted in a few locations around Southern Shara.
|Southern Arun||Southern Shara||Northern Shara|
|Arcadia (11-23)||Allemantheia Outskirts (Allemantheia)||Helkan District (59-60)|
|Baldera (TBA)||Essenia (40-42)||Kaiator Outskirts (Kaiator)|
|Island of Dawn (1-11)||Val Elenium (54-60)||Lorcada (56-59)|
|Ostgarath (31-33, 48-51)||Val Palrada (45-48)||Sylvanoth (51-54, 58-69)|
|Poporia (24-27, 34-35)||Veritas District (60)||Val Kaeli (60)|
|Val Aureum (28-31, 35-38)||Westonia (42-47)||Val Tirkai (59-60)|
|Velika Plains (Velika)|