In the beginning there were three realms: God realm, Sura realm and Human realm. Gods and Suras were free to access each realm, and at that time humans could use magic by invoking a God's or a Sura's power.
But following the events of the N0 Cataclysm, Visnu separated the races into the three realms, making impossible for the Gods and superior Suras to set foot on the Human realm ever again, without a summoner to call for them (Kubera is the only exception).
The Human realm is supposed to be the best place among the three realms, inhabitable and with a rich environment. It's composed by many inhabited planets, filled with villages, towns and cities, and the latter are the most safe place to live, being protected by barriers. The world, in fact, is not populated only by pureblood humans, but by Halfs, Quarters and inferior Suras too. The Upani and Mara Suras are the main reason for the barriers' existence.