The world map is one of several preselected maps. After Oryx summons the players to battle him, the server changes to a different map. Different game servers will be using different maps at any given time.
This map shows the outline of one of the current worlds and all 3 roads:
The maps are islands and include:
In this area, the enemies pose minimal threat. They don't hit hard and their attacks are easy to dodge. Beach is the only biome found in this area. This area includes a single type of wandering quest enemy.
In this area, enemies are relatively easy too, but are more numerous. In certain locations, swarms of enemies can be seen. This biome is divided from driest to wettest, into desert, grassland, forest and a swamp. All quest enemies found in this elevation are wandering quest enemies.
In this area, the enemies are the most erratic and it can be very hard to dodge their attacks. Being careless in this region can be fatal for lone low-level adventurers but it's still acceptable even with basic equipment to fight enemies. Reasonably well-equipped level-20 players will have little trouble in this area. This area includes both wandering quest enemies and quest area enemies. Biomes in this region are from driest to wettest, desert, savannah, grasslands and forests. Desert biome is rather rare in this area.
While the enemies in this area have attacks that are not as erratic as in midlands and it's possible to dodge the enemies when being careful, it's easy to die since the enemies pack a lot of punch and a proper equipment is recommended to travel alone. All quest enemies in this area are located in structures specific to them. Biomes in this region are the same as in midlands except the desert is rare.
This area has got the strongest enemies and going alone will likely lead to the death. Enemies in this region usually have elaborate bullet patterns. The mountains include a snowy area and rocky area (also known as godlands). There may be lava in this area.
Build 104 features newer maps than build 100. Terrain features are generally smoother and less angular, with a more gradual transition between different terrain types. The shape of the landmasses is more complex, with more bays and peninsulas. Roads and rivers are improved. The world maps are generated every few months, and every time Oryx destroys the world one of the 20 maps is randomly chosen for the new world.
The creation of geographical features (but not quest areas, vegetation, terrain types, or monsters) for build 100 and up is described in these blog posts:
Build 92 through 99 had twenty maps in rotation. Earlier builds had a single map. These maps were created using Perlin Noise. See amitp's post on map creation for more information.