Example of Overworld
The Overworld is the starting dimension in Minecraft. As with all dimensions in the game, the Overworld can generate on the horizontal plane to a limited 862 by 862 blocks. All blocks beyond this limit are "fake blocks", and can not be interacted with, or stood on. This is usually called the end of the world.
Overall Appearance
The Overworld encompasses the level ceiling down to bedrock and extending in every direction on the horizontal plane. It is generated through a secret process which creates multiple Noise maps to create differing elevations, general chunk shapes, and complex mountain and cave systems like Ravines or Abandoned Mineshafts.
Mobs
Most mobs in the game can appear in the Overworld: Wolves, Pigs, Sheep, Cows, Mooshrooms, Chickens, Squid, Zombies, Skeletons, Spiders, NPC Villagers, Spider Jockeys, Endermen, Creepers and Slimes all spawn normally, and Pigs can, very rarely, be hit by lightning, which will turn them into Zombie Pigmen. Zombie Pigmen can also spawn near nether portals. Also, if Creepers get hit by lightning, they can turn into Charged Creepers, though this happens very rarely.
Mobs from The Nether may also pass through Nether Portals and enter the Overworld in the PC version, the Zombie Pigman, the Magma Cube, and the Blaze (mobs from the overworld can also enter the Nether through a Nether Portal). The Ender Dragon however can't enter the overworld because there is no return portal from The End unless the dragon is defeated.
Structures
Villages are a type of structure in Minecraft that includes Villagers that trade with you, and where Zombie Sieges occur. Dungeons are underground structures that have 1 small room which has a block called a Monster Spawner which spawns a monster every few seconds (10 to 40 seconds) Ravines are big caves that most of the time cut through the landscape above them. Abandoned Mineshafts are structures that consists of cobwebs, rails, cave spider spawners, and wooden supports (Wood planks and Wood Fences). Lastly, Strongholds are where you find the End Portal and fight the Enderdragon.