Dimensions
Dimensions are what Minecraft calls its different worlds that the player can explore, usually via portals. They often have wildly different terrain, mobs and survival strategies.
Overworld
The primary world of Minecraft. It is given the numerical Id 0. It's terrain data is stored in the world folder as a result.
The overworld features:
- Earth-like terrain generation
- Various biomes
- Water oceans
- Progessing time of day
- Changing Weather (Rain, Thunder)
- Can spawn Pigs, Chickens, Cows, Sheep, Squids and Wolves
- Can spawn Zombies, Creepers, Skeletons and Spiders
Nether
The secondary world of Minecraft, inspired by hell. It is given the numerical Id -1. It's terrain data is stored in world/DIM-1 as a result.
The nether features:
- Cavern-like terrain generation
- Only one biome, "hell"
- Lava oceans
- Can spawn Zombie Pigmen and Ghasts
- Has spontaneously spawning fire
- Water evaporation, meaning it cannot be placed (without exploits)
- Faster lava flow
Sky
The sky dimension is the unfinished dimension that later became the End. Its numeric ID is 1. Beta 1.7.3 contains the Sky dimension, biome, and fully working terrain generation code, but there is no way to reach it without client or server modifications.
You can pass 1 to the Login and Respawn packets to have the client join it.
The sky dimension does not implement saving logic, therefore it does not have its own folder like DIM1.
Screenshot of the Sky dimension. Source: Minecraft Wiki.