Points of Light Setting Concept.
This is directly lifted from the WotC article:
The Dungeons & Dragons game assumes many things about its setting: The world is populated by a variety of intelligent races, strange monsters lurk on other planes, ancient empires have left ruins across the face of the world, and so on. But one of the new key concepts about the D&D world is simply this: Civilized folk live in small, isolated points of light scattered across a big, dark, dangerous world.
Most of the world is monster-haunted wilderness. The centers of civilization are few and far between, and the world isn’t carved up between nation-states that jealously enforce their borders. A few difficult and dangerous roads tenuously link neighboring cities together, but if you stray from them you quickly find yourself immersed in goblin-infested forests, haunted barrowfields, desolate hills and marshes, and monster-hunted badlands. Anything could be waiting down that old overgrown dwarf-built road: a den of ogre marauders, a forgotten tower where a lamia awaits careless travelers, a troll’s cave, a lonely human village under the sway of a demonic cult, or a black wood where shadows and ghosts thirst for the blood of the living.
Given the perilous nature of the world around the small islands of civilization, many adventures revolve around venturing into the wild lands. For example: Roads are often closed by bandits, marauders such as goblins or gnolls, or hungry monsters such as griffons or dragons. The simple mission of driving off whomever or whatever is preying on unfortunate travelers is how many young heroes begin their careers.
Since towns and villages do not stay in close contact, it’s easy for all sorts of evils to befall a settlement without anyone noticing for a long time. A village might be terrorized by a pack of werewolves or enslaved by an evil wizard, and no one else would know until adventurers stumbled into the situation.
Many small settlements and strongholds are founded, flourish for a time, and then fall into darkness. The wild lands are filled with forgotten towers, abandoned towns, haunted castles, and ruined temples. Even people living only a few miles away from such places might know them only by rumor and legend.
The common folk of the world look upon the wild lands with dread. Few people are widely traveled—even the most ambitious merchant is careful to stick to better-known roads. The lands between towns or homesteads are wide and empty. It might be safe enough within a day’s ride of a city or an hour’s walk of a village, but go beyond that and you are taking your life into your hands. People are scared of what might be waiting in the old forest or beyond the barren hills at the far end of the valley, because whatever is out there is most likely hungry and hostile. Striking off into untraveled lands is something only heroes and adventurers do.
Another implication of this basic concept of the world is that there is very little in the way of authority to deal with raiders and marauders, outbreaks of demon worship, rampaging monsters, deadly hauntings, or similar local problems. Settlements afflicted by troubles can only hope for a band of heroes to arrive and set things right. If there is a kingdom beyond the town’s walls, it’s still largely covered by unexplored forest and desolate hills where evil folk gather. The king’s soldiers might do a passable job of keeping the lands within a few miles of his castle free of monsters and bandits, but most of the realm’s outlying towns and villages are on their own.
In such a world, adventurers are aberrant. Commoners view them as brave at best, and insane at worst. But such a world is rife with the possibility for adventure, and no true hero will ever lack for a villain to vanquish or a quest to pursue.